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=9c6148d3e5a792e1f6e77ab901789811c0a4854c&amp;showmsg=1'>root</a>/<a href='/guix/log/gnu?id=9c6148d3e5a792e1f6e77ab901789811c0a4854c&amp;showmsg=1'>gnu</a>/<a href='/guix/log/gnu/packages?id=9c6148d3e5a792e1f6e77ab901789811c0a4854c&amp;showmsg=1'>packages</a>/<a href='/guix/log/gnu/packages/elixir.scm?id=9c6148d3e5a792e1f6e77ab901789811c0a4854c&amp;showmsg=1'>elixir.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/elixir.scm?id=9c6148d3e5a792e1f6e77ab901789811c0a4854c'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2021-03-18 17:08:10 +0100'>2021-03-18</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/elixir.scm?id=c2265e0285f82ac32b77116e6c1b476461241390'>gnu: elixir: Update to 1.11.4.</a></td><td>Tobias Geerinckx-Rice</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/packages/elixir.scm (elixir): Update to 1.11.4.


</td></tr>
<tr class='logheader'><td><span title='2021-01-19 21:37:04 +0200'>2021-01-19</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/elixir.scm?id=d485ef5ce9639c7e4d296a476b246e8b85e8f639'>gnu: elixir: Update to 1.11.3.</a></td><td>Oskar Köök</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/packages/elixir.scm (elixir): Update to 1.11.3.

Signed-off-by: Efraim Flashner &lt;efraim@flashner.co.il&gt;


</td></tr>