nixos/systemd: disable timer units with service

A disabled systemd service with a "startAt" attribute, like this:

  systemd.services.foo-service = {
    enable = false;
    startAt = "*-*-* 05:15:00";
    ...
  };

will cause the following errors in the system journal:

  systemd[1]: foo-service.timer: Refusing to start, unit to trigger not loaded.
  systemd[1]: Failed to start foo-service.timer.

Fix it by not generating the corresponding timer unit when the service
is disabled.

nixos/modules/system/boot/systemd.nix

@@ -764,7 +764,7 @@ in
         { wantedBy = [ "timers.target" ];
           timerConfig.OnCalendar = service.startAt;
         })
-        (filterAttrs (name: service: service.startAt != "") cfg.services);
+        (filterAttrs (name: service: service.enable && service.startAt != "") cfg.services);
 
     # Generate timer units for all services that have a ‘startAt’ value.
     systemd.user.timers =