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=76177e816af239e32f9df7a2e764a7a1084cfc00'>root</a>/<a href='/guix/log/tests?id=76177e816af239e32f9df7a2e764a7a1084cfc00'>tests</a>/<a href='/guix/log/tests/crate.scm?id=76177e816af239e32f9df7a2e764a7a1084cfc00'>crate.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/crate.scm?id=76177e816af239e32f9df7a2e764a7a1084cfc00&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-01-13 22:42:49 +0100'>2021-01-13</span></td><td><a href='/guix/commit/tests/crate.scm?id=23862011c3d02814181399765a024c7fa3fadd97'>tests: Fix JSON syntax error in 'crate.scm'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-04 16:59:26 +0100'>2020-12-04</span></td><td><a href='/guix/commit/tests/crate.scm?id=e3065ec11742eef58ca688db240d86031941a39a'>import: crate: Skip tests when Guile-Semver is missing.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-02 22:09:23 +0100'>2020-12-02</span></td><td><a href='/guix/commit/tests/crate.scm?id=054e308f5d85ca96327861a577d69c6e18fdc9dc'>import: crate: Use existing package satisfying semver requirement.</a><span class='msg-avail'>...</span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2020-12-02 22:09:23 +0100'>2020-12-02</span></td><td><a href='/guix/commit/tests/crate.scm?id=45584061a9ebe961e2be08ee94c3fe8ad74e2713'>import: crate: Trim version for names after left-most non-zero part.</a><span class='msg-avail'>...</span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2020-12-02 22:09:23 +0100'>2020-12-02</span></td><td><a href='/guix/commit/tests/crate.scm?id=9a48e35be853e10ba9b21bb91ef52a66b4264c84'>import: utils: Trim patch version from names.</a><span class='msg-avail'>...</span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2020-12-02 22:09:23 +0100'>2020-12-02</span></td><td><a href='/guix/commit/tests/crate.scm?id=50fbb3f032b46a565ca225daddf4eeb7c8edfab4'>import: crate: Parameterized importing of dev dependencies.</a><span class='msg-avail'>...</span></td><td>Martin Becze</td></tr>
<tr><td><span title='2020-12-02 22:09:23 +0100'>2020-12-02</span></td><td><a href='/guix/commit/tests/crate.scm?id=269c1db41bd82f93c7ae5c62a4969a423e556183'>import: crate: Use guile-semver to resolve module versions.</a><span class='msg-avail'>...</span></td><td>Martin Becze</td></tr>
<tr><td><span title='2020-04-27 23:29:40 +0200'>2020-04-27</span></td><td><a href='/guix/commit/tests/crate.scm?id=5dfe02c60767a633c67f7f6fc9557b54b3c99b63'>tests: Remove trailing commas in JSON tests.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-16 23:14:05 +0100'>2020-01-16</span></td><td><a href='/guix/commit/tests/crate.scm?id=72c678af55390ce01bec590f760ab95af67663b2'>import: crate: Export 'string-&gt;license'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-12-11 12:13:03 +0100'>2019-12-11</span></td><td><a href='/guix/commit/tests/crate.scm?id=263a267b75e472cb84428571580dabd99d5dff0c'>import: crate: Better handle license expressions.</a><span class='msg-avail'>...</span></td><td>Brice Waegeneire</td></tr>
<tr><td><span title='2019-12-11 12:13:03 +0100'>2019-12-11</span></td><td><a href='/guix/commit/tests/crate.scm?id=4982de4c3299f367ab582e0865c39186a12ab3a1'>import: crate: Add recursive import test.</a><span class='msg-avail'>...</span></td><td>Brian Leung</td></tr>