nixos/gdm: add gdm to XDG_DATA_DIRS

This is necessary so that gnome-session can find GDM's gnome-login.session,
see https://gitlab.gnome.org/GNOME/gdm/-/issues/756

nixos/modules/services/x11/display-managers/gdm.nix

@@ -149,7 +149,7 @@ in
         environment = {
           GDM_X_SERVER_EXTRA_ARGS = toString
             (filter (arg: arg != "-terminate") cfg.xserverArgs);
-          XDG_DATA_DIRS = "${cfg.sessionData.desktops}/share/";
+          XDG_DATA_DIRS = "${gdm}/share:${cfg.sessionData.desktops}/share";
         } // optionalAttrs (xSessionWrapper != null) {
           # Make GDM use this wrapper before running the session, which runs the
           # configured setupCommands. This relies on a patched GDM which supports