nixos/tzupdate: make enabled module actually be enabled

Without this fix, when setting `services.tzupdate.enable = true`, the service would never run automatically.

Now, it's actually enabled in systemd and it actually gets executed.

Still, it could be improved with a timer as explained in NixOS#127984 (comment), but this makes it at least work out of the box when rebooting the system.

nixos/modules/services/misc/tzupdate.nix

@@ -20,11 +20,12 @@ in {
     # zone, which is better than silently overriding it.
     time.timeZone = null;
 
-    # We provide a one-shot service which can be manually run. We could
-    # provide a service that runs on startup, but it's tricky to get
-    # a service to run after you have *internet* access.
+    # We provide a one-shot service that runs at startup once network
+    # interfaces are up, but we can’t ensure we actually have Internet access
+    # at that point. It can also be run manually with `systemctl start tzupdate`.
     systemd.services.tzupdate = {
       description = "tzupdate timezone update service";
+      wantedBy = [ "multi-user.target" ];
       wants = [ "network-online.target" ];
       after = [ "network-online.target" ];
       script = ''