nixos/tzupdate: Fix reference to non-existing attribute (#402226)

nixos/modules/services/misc/tzupdate.nix

···
 
     systemd.timers.tzupdate = {
       enable = cfg.timer.enable;
-      interval = cfg.timer.interval;
       timerConfig = {
         OnStartupSec = "30s";
         OnCalendar = cfg.timer.interval;

nixos/tests/all-tests.nix

···
   tuxguitar = runTest ./tuxguitar.nix;
   twingate = runTest ./twingate.nix;
   typesense = handleTest ./typesense.nix { };
+  tzupdate = runTest ./tzupdate.nix;
   ucarp = handleTest ./ucarp.nix { };
   udisks2 = handleTest ./udisks2.nix { };
   ulogd = handleTest ./ulogd/ulogd.nix { };

nixos/tests/tzupdate.nix

···
+{ lib, ... }:
+let
+  clientNodeName = "client";
+in
+{
+  name = "tzupdate";
+
+  # TODO: Test properly:
+  # - Add server node
+  # - Add client configuration to talk to the server node
+  # - Assert that the time zone changes appropriately
+  nodes.${clientNodeName} = {
+    services.tzupdate.enable = true;
+  };
+
+  testScript = ''
+    start_all()
+    ${clientNodeName}.wait_for_unit("multi-user.target")
+  '';
+
+  meta.maintainers = [ lib.maintainers.l0b0 ];
+}