cron: set TZDIR so cron is able to find the TZ

modules/services/scheduling/cron.nix

@@ -91,7 +91,10 @@ in
         startOn = "startup";
 
         # Needed to interpret times in the local timezone.
-        environment = { TZ = config.time.timeZone; };
+        environment = {
+          TZ = config.time.timeZone;
+          TZDIR = "${pkgs.tzdata}/share/zoneinfo";
+        };
 
         path = [ cronNixosPkg ];