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=eeaeac23549597cec0d445bafc4e9f94f79910c3'>root</a>/<a href='/guix/log/etc?id=eeaeac23549597cec0d445bafc4e9f94f79910c3'>etc</a>/<a href='/guix/log/etc/system-tests.scm?id=eeaeac23549597cec0d445bafc4e9f94f79910c3'>system-tests.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/etc/system-tests.scm?id=eeaeac23549597cec0d445bafc4e9f94f79910c3&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-03-23 00:06:58 +0100'>2020-03-23</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=8b18f5cb309f670b08ad35df6425f4e9e19d8a82'>tests: Use the same source as ci.guix.gnu.org.</a><span class='msg-avail'>...<span class='msg-tooltip'>* etc/system-tests.scm (system-test-manifest): In 'local-file' call, use
a name that is a function of COMMIT to match what Cuirass does.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-05 16:33:38 +0100'>2020-03-05</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=5ec4156bbcaec8337f78411204d59e59e706103b'>tests: Add a manifest for system tests.</a><span class='msg-avail'>...<span class='msg-tooltip'>The manifest can be passed to 'guix build -m', 'guix weather -m', and so
on.  It can also be passed to an installed 'guix' (without
./pre-inst-env), with the exception so far of installation tests.

* build-aux/run-system-tests.scm: Remove.  Move interesting bits move
to...
* etc/system-tests.scm: ... here.  New file.
* Makefile.am (EXTRA_DIST): Remove 'build-aux/run-system-tests.scm' and
add 'etc/system-tests.scm'.
(check-system): Rewrite to run 'guix build -m etc/system-tests.scm'.
</span></span></td><td>Ludovic Courtès</td></tr>