nixos/localtimed: set time.timeZone to null to avoid silent overriding

Currently if a timezone was selected explicitly, the service will
silently override the value, essentially ignoring what is meant to be a
a deliberate choice of option. This may cause confusion as to why the
option is not doing anything when this service is enabled, particularly
in more complex set-ups after some time.

This will simply make the choice deliberate from the user's part, either
by having to remove the option or lowering its priority as a recognition
that it may be ignored.

This change was inspired by the `services.tzupdate` module, which does
the same.

[1]: <https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/misc/tzupdate.nix#L24>

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -653,6 +653,9 @@
 
 - ZFS now imports its pools in `postResumeCommands` rather than `postDeviceCommands`. If you had `postDeviceCommands` scripts that depended on ZFS pools being imported, those now need to be in `postResumeCommands`.
 
+- `services.localtimed.enable = true` will now set `time.timeZone = null`.
+  This is to avoid silently shadowing a user's explicitly defined timezone without recognition on the user's part.
+
 ## Detailed migration information {#sec-release-24.11-migration}
 
 ### `sound` options removal {#sec-release-24.11-migration-sound}

nixos/modules/services/system/localtimed.nix

@@ -16,6 +16,11 @@ in {
           Enable `localtimed`, a simple daemon for keeping the
           system timezone up-to-date based on the current location. It uses
           geoclue2 to determine the current location.
+
+          To avoid silent overriding by the service, if you have explicitly set a
+          timezone, either remove it or ensure that it is set with a lower priority
+          than the default value using `lib.mkDefault` or `lib.mkOverride`. This is
+          to make the choice deliberate. An error will be presented otherwise.
         '';
       };
       package = mkPackageOption pkgs "localtime" { };
@@ -24,6 +29,10 @@ in {
   };
 
   config = mkIf cfg.enable {
+    # This will give users an error if they have set an explicit time
+    # zone, rather than having the service silently override it.
+    time.timeZone = null;
+
     services.geoclue2.appConfig.localtimed = {
       isAllowed = true;
       isSystem = true;