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=7eb9176a6e5b116bc244a08ddd72d15cfeeb3ba0'>root</a>/<a href='/guix/log/gnu?id=7eb9176a6e5b116bc244a08ddd72d15cfeeb3ba0'>gnu</a>/<a href='/guix/log/gnu/packages?id=7eb9176a6e5b116bc244a08ddd72d15cfeeb3ba0'>packages</a>/<a href='/guix/log/gnu/packages/nim.scm?id=7eb9176a6e5b116bc244a08ddd72d15cfeeb3ba0'>nim.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/gnu/packages/nim.scm?id=7eb9176a6e5b116bc244a08ddd72d15cfeeb3ba0&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-01-23 18:35:47 +0100'>2022-01-23</span></td><td><a href='/guix/commit/gnu/packages/nim.scm?id=1c2090ddec0ba9cae09a26de93e123b9c153b6a7'>gnu: Update nim to 1.6.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nim.scm(nim): Update to 1.6.2.

Signed-off-by: Nicolas Goaziou &lt;mail@nicolasgoaziou.fr&gt;
</span></span></td><td>(unmatched-parenthesis via Guix-patches via</td></tr>
<tr><td><span title='2021-05-02 03:00:04 +0200'>2021-05-02</span></td><td><a href='/guix/commit/gnu/packages/nim.scm?id=0cf79dd998e82f0420da6849a29b4cb9b6f18921'>gnu: nim: Update to 1.4.6.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nim.scm (nim): Update to 1.4.6.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-02-11 11:30:49 +0800'>2021-02-11</span></td><td><a href='/guix/commit/gnu/packages/nim.scm?id=6fbefd03f0433ed4777ffd041746e32535ccc69f'>gnu: nim: Update to 1.4.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nim.scm (nim): Update to 1.4.2.

Signed-off-by: 宋文武 &lt;iyzsong@member.fsf.org&gt;
</span></span></td><td>Yuriy Glukhov</td></tr>
<tr><td><span title='2020-04-26 13:31:16 +0200'>2020-04-26</span></td><td><a href='/guix/commit/gnu/packages/nim.scm?id=52bdd30407756ca883dc98cc3ed776acd740f14c'>gnu: nim: Adjust for Guile 3.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nim.scm (nim)[arguments]: Provide explicit destination in calls
to FORMAT.
</span></span></td><td>Marius Bakke</td></tr>