Use $TZDIR to search for time-zone data. Thus avoid depending on package
"tzdata", which often introduces changes with near-immediate effects, so it's
important to be able to update it fast.

Based on a patch fron NixOS.
===================================================================
--- qtbase-opensource-src-5.14.2.orig/src/corelib/time/qtimezoneprivate_tz.cpp
+++ qtbase-opensource-src-5.15.2/src/corelib/time/qtimezoneprivate_tz.cpp
@@ -70,7 +70,11 @@
 // Parse zone.tab table, assume lists all installed zones, if not will need to read directories
 static QTzTimeZoneHash loadTzTimeZones()
 {
-    QString path = QStringLiteral("/usr/share/zoneinfo/zone.tab");
+    // Try TZDIR first, in case we're running on GuixSD.
+    QString path = QFile::decodeName(qgetenv("TZDIR")) + QStringLiteral("/zone.tab");
+    // Fallback to traditional paths in case we are not on GuixSD.
+    if (!QFile::exists(path))
+        path = QStringLiteral("/usr/share/zoneinfo/zone.tab");
     if (!QFile::exists(path))
         path = QStringLiteral("/usr/lib/zoneinfo/zone.tab");
 
@@ -645,6 +649,9 @@
         if (!tzif.open(QIODevice::ReadOnly))
             return;
     } else {
+      // Try TZDIR first, in case we're running on GuixSD.
+      tzif.setFileName(QFile::decodeName(qgetenv("TZDIR")) + QStringLiteral("/") + QString::fromLocal8Bit(ianaId));
+      if (!tzif.open(QIODevice::ReadOnly)) {
         // Open named tz, try modern path first, if fails try legacy path
         tzif.setFileName(QLatin1String("/usr/share/zoneinfo/") + QString::fromLocal8Bit(ianaId));
         if (!tzif.open(QIODevice::ReadOnly)) {
@@ -652,6 +659,7 @@
             if (!tzif.open(QIODevice::ReadOnly))
                 return;
         }
+      }
     }
 
     QDataStream ds(&tzif);
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<div class='path'>path: <a href='/guix/log/?id=ccf64b6a8b8718a8bb69719cf9ed2873464e3850'>root</a>/<a href='/guix/log/tests?id=ccf64b6a8b8718a8bb69719cf9ed2873464e3850'>tests</a>/<a href='/guix/log/tests/challenge.scm?id=ccf64b6a8b8718a8bb69719cf9ed2873464e3850'>challenge.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/challenge.scm?id=ccf64b6a8b8718a8bb69719cf9ed2873464e3850&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-01-16 11:51:03 +0000'>2021-01-16</span></td><td><a href='/guix/commit/tests/challenge.scm?id=681af17460fa34916ac40589cccdcca7222249af'>guix: Move narinfo code from substitute script to module.</a><span class='msg-avail'>...<span class='msg-tooltip'>This separation between the code for dealing with narinfos from the code doing
that for a purpose should make things clearer, and better support components
other that the substitute script in using this code.

This is just moving the code around, no code should have been significantly
changed.

* guix/scripts/substitute.scm (&lt;narinfo&gt;): Move record type to (guix narinfo).
(fields-&gt;alist, narinfo-hash-algorithm+value, narinfo-hash-&gt;sha256,
narinfo-signature-&gt;canonical-sexp, narinfo-maker, read-narinfo,
narinfo-sha256, valid-narinfo?, write-narinfo, narinfo-&gt;string,
string-&gt;narinfo, equivalent-narinfo?, supported-compression?,
compresses-better?, narinfo-best-uri): Move procedures to (guix narinfo).
(%compression-methods): Move variable to (guix narinfo).
* guix/narinfo.scm: New file.
* Makefile.am (MODULES): Add it.
* po/guix/POTFILES.in: Add 'guix/narinfo.scm'.
</span></span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2020-05-22 01:29:39 +0200'>2020-05-22</span></td><td><a href='/guix/commit/tests/challenge.scm?id=ce0be5675b702b2ff89aed1772ebb42af4150243'>packages: Introduce &lt;content-hash&gt; and use it in &lt;origin&gt;.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/packages.scm (&lt;content-hash&gt;): New record type.
(define-content-hash-constructor, build-content-hash)
(content-hash): New macros.
(print-content-hash): New procedure.
(&lt;origin&gt;): Rename constructor to '%origin'.
[sha256]: Remove field.
[hash]: New field.  Adjust users.
(origin-compatibility-helper, origin): New macros.
(origin-sha256): New deprecated procedure.
(origin-&gt;derivation): Adjust accordingly.
* tests/packages.scm ("package-source-derivation, origin, sha512"): New
test.
* guix/tests.scm: Hide (gcrypt hash) 'sha256' for proper syntax
matching.
* tests/challenge.scm: Add #:prefix for (gcrypt hash) and adjust users.
* tests/derivations.scm: Likewise.
* tests/store.scm: Likewise.
* tests/graph.scm ("bag DAG, including origins"): Provide 'sha256' field
with the right length.
* gnu/packages/aspell.scm (aspell-dictionary)
(aspell-dict-ca, aspell-dict-it): Use 'hash' and 'content-hash' for
proper syntax matching.
* gnu/packages/bash.scm (bash-patch): Rename 'sha256' to 'sha256-bv'.
* gnu/packages/bootstrap.scm (bootstrap-executable): Rename 'sha256' to 'bv'.
* gnu/packages/readline.scm (readline-patch): Likewise.
* gnu/packages/virtualization.scm (qemu-patch): Rename 'sha256' to
'sha256-bv'.
* guix/import/utils.scm: Hide (gcrypt hash) 'sha256'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-12-12 17:56:58 +0100'>2019-12-12</span></td><td><a href='/guix/commit/tests/challenge.scm?id=828a39da68a9169ef1d9f9ff02a1c66b1bcbe884'>challenge: Support "--diff=diffoscope".</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/challenge.scm (call-with-nar): New procedure.
(narinfo-contents): Express in terms of 'call-with-nar'.
(call-with-mismatches, report-differing-files/external): New
procedures.
(%diffoscope-command): New variable.
(%options): Support "diffoscope" and a string starting with "/".
* tests/challenge.scm (call-mismatch-test): New procedure.
("differing-files"): Rewrite in terms of 'call-mismatch-test'.
("call-with-mismatches"): New test.
* doc/guix.texi (Invoking guix challenge): Document it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-12-12 17:56:58 +0100'>2019-12-12</span></td><td><a href='/guix/commit/tests/challenge.scm?id=5208db3a526e3fcdb8473d9bab8afe498c5f3f76'>challenge: Add "--diff".</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/challenge.scm (dump-port*): New variable.
(archive-contents, store-item-contents, narinfo-contents)
(differing-files, report-differing-files): New procedures.
(summarize-report): Add #:report-differences and call it.
(show-help, %options): Add "--diff".
(%default-options): Add 'difference-report' key.
(report-differing-files): Parameterize CURRENT-TERMINAL-COLUMNS and pass
 #:report-differences to 'summarize-report'.
* guix/tests/http.scm (%local-url): Add optional argument.
(call-with-http-server): Fix docstring typo.
* tests/challenge.scm (query-path-size, make-narinfo): New procedures.
("differing-files"): New test.
* doc/guix.texi (Invoking guix challenge): Document "--diff".
</span></span></td><td>Ludovic Courtès</td></tr>