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);
value='author'>author</option>
<option value='committer'>committer</option>
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<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=24a271684e95d80982c0a1250255215a278ee171'>root</a>/<a href='/guix/log/gnu?id=24a271684e95d80982c0a1250255215a278ee171'>gnu</a>/<a href='/guix/log/gnu/services?id=24a271684e95d80982c0a1250255215a278ee171'>services</a>/<a href='/guix/log/gnu/services/rsync.scm?id=24a271684e95d80982c0a1250255215a278ee171'>rsync.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/services/rsync.scm?id=24a271684e95d80982c0a1250255215a278ee171&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-11-26 23:34:48 +0100'>2023-11-26</span></td><td><a href='/guix/commit/gnu/services/rsync.scm?id=da2dc98185526796fbfb4bc63caf9d70aead9037'>services: rsync: Provide a prettier Shepherd inetd service name.</a><span class='msg-avail'>...<span class='msg-tooltip'>Previously, a default name would be used for transient inetd services,
leading to names such as #{5rjnskb6011jgmfmpgp63jp9mh9nkl20-rsync-3274}#.

* gnu/services/rsync.scm (rsync-shepherd-service): Pass #:service-name-stem.

Change-Id: I4d0a4a01e122c6eeaa85f104b083cf7e8709eac3
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-05-18 23:11:19 -0400'>2023-05-18</span></td><td><a href='/guix/commit/gnu/services/rsync.scm?id=d43d8377c741c849ca89dccb07e7d87f82ae88d5'>services: rsync: Use least authority wrapper.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/rsync.scm (rsync-shepherd-service) Wrap rsync command in a
least-authority-wrapper.

Reviewed-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2023-05-18 23:10:56 -0400'>2023-05-18</span></td><td><a href='/guix/commit/gnu/services/rsync.scm?id=03e601da49e43ed0235144f32095ce5f58f16275'>services: rsync: Use make-inetd-constructor.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/rsync.scm (rsync-shepherd-service): Use make-inetd-constructor
if available in start slot.
* gnu/tests/rsync.scm (run-rsync-test): Delete "PID file" test.

Reviewed-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2023-04-21 16:16:37 +0200'>2023-04-21</span></td><td><a href='/guix/commit/gnu/services/rsync.scm?id=70677d882276ff44bc31d39e378749b976a9ca7d'>services: rsync: Add 'configuration' action.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/rsync.scm (rsync-shepherd-service): Add 'actions' field.
</span></span></td><td>Ludovic Courtès</td></tr>