modules/redshift-gammastep: install package into the profile (#3710)

This is needed to make sure that the tray icon is actually found.
Otherwise an empty icon indicating that it wasn't found is shown.

modules/services/redshift-gammastep/lib/options.nix

@@ -183,6 +183,8 @@ in {
     xdg.configFile.${xdgConfigFilePath}.source =
       settingsFormat.generate xdgConfigFilePath cfg.settings;
 
+    home.packages = [ cfg.package ];
+
     systemd.user.services.${moduleName} = {
       Unit = let
         geoclueAgentService =