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-5.9.4.orig/src/corelib/tools/qtimezoneprivate_tz.cpp
+++ qtbase-opensource-src-5.9.4/src/corelib/tools/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=0e407e53dfb5f6bf7bc57cbe7ffa46db1af45524'>root</a>/<a href='/guix/log/po?id=0e407e53dfb5f6bf7bc57cbe7ffa46db1af45524'>po</a>/<a href='/guix/log/po/doc?id=0e407e53dfb5f6bf7bc57cbe7ffa46db1af45524'>doc</a>/<a href='/guix/log/po/doc/guix-cookbook.de.po?id=0e407e53dfb5f6bf7bc57cbe7ffa46db1af45524'>guix-cookbook.de.po</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/po/doc/guix-cookbook.de.po?id=0e407e53dfb5f6bf7bc57cbe7ffa46db1af45524&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-01-09 11:35:59 +0100'>2022-01-09</span></td><td><a href='/guix/commit/po/doc/guix-cookbook.de.po?id=934b0cdfee2078bad42429427e12d6ee2ddb7fec'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-cookbook.fi.po: New file.
* po/doc/guix-manual.fi.po: New file.
* po/doc/local.mk: Add them.
* doc/local.mk: Add them.
* po/guix/fi.po: New file.
* po/guix/LINGUAS: Add it.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-12-04 19:40:52 +0100'>2021-12-04</span></td><td><a href='/guix/commit/po/doc/guix-cookbook.de.po?id=7fcb20a8fa242ab53b9bc97ef0b0367c6a399bd4'>nls: Update translations.</a></td><td>Julien Lepiller</td></tr>
not call
POPULATE-SINGLE-PROFILE-DIRECTORY, which creates extraneous files such as
/root.  Instead, call POPULATE-STORE and INSTALL-DATABASE-AND-GC-ROOTS
individually to more precisely generate the file system.  Replace the list of
files by the current directory, "." and streamline the way options are passed.
* gnu/system/file-systems.scm (reduce-directories): Remove procedure.
* tests/file-systems.scm ("reduce-directories"): Remove test.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-06-29 14:53:21 -0400'>2021-06-29</span></td><td><a href='/guix/commit/tests/file-systems.scm?id=4f3bdc8f21657dbda857027b3ec8754dd4c7c67b'>pack: Prevent duplicate files in tar archives.</a><span class='msg-avail'>...<span class='msg-tooltip'>Tar translate duplicate files in the archive into hard links.  These can cause
problems, as not every tool support them; for example dpkg doesn't.

* gnu/system/file-systems.scm (reduce-directories): New procedure.
(file-prefix?): Lift the restriction on file prefix.  The procedure can be
useful for comparing relative file names.  Adjust doc.
(file-name-depth): New procedure, extracted from ...
(btrfs-store-subvolume-file-name): ... here.
* guix/scripts/pack.scm (self-contained-tarball/builder): Use
reduce-directories.
* tests/file-systems.scm ("reduce-directories"): New test.
</span></span></td><td>Maxim Cournoyer</td></tr>