cron service: fix reliance on etc.timezone

This does not fully address the issue, as cron will not restart when
the timezone is changed imperatively as it presumably needs to.

nixos/modules/services/scheduling/cron.nix

@@ -122,7 +122,7 @@ in
               fi
             '';
 
-          restartTriggers = [ config.environment.etc.localtime.source ];
+          restartTriggers = [ config.time.timeZone ];
           serviceConfig.ExecStart = "${cronNixosPkg}/bin/cron -n";
         };