nixos/udev: only restart when udev rules change

Only restart `systemd-udevd.service` if udev rules in `/etc/udev/rules`
actually changed. The paths of `services.udev.packages` may change
frequently but the resulting udev rules built by `udevRulesFor`
likely change less often.

This has the added benefit of not adding `services.udev.packages` to
the system closure if they are only used for their udev rules
(issue #308937).

nixos/modules/services/hardware/udev.nix

@@ -434,7 +434,7 @@ in
     '';
 
     systemd.services.systemd-udevd =
-      { restartTriggers = cfg.packages;
+      { restartTriggers = [ config.environment.etc."udev/rules.d".source ];
       };
 
   };