nixos/tests/flatpak: explicitly disable gdm to fix eval

The switch to lightdm as default display manager in #30890
broke eval of the flatpak test. Since the test uses the
auto display manager (lightdm), gdm must now be explicitly disabled.

nixos/tests/flatpak.nix

···
   machine = { pkgs, ... }: {
     imports = [ ./common/x11.nix ];
     services.xserver.desktopManager.gnome3.enable = true; # TODO: figure out minimal environment where the tests work
+    # common/x11.nix enables the auto display manager (lightdm)
+    services.xserver.displayManager.gdm.enable = false;
     environment.gnome3.excludePackages = pkgs.gnome3.optionalPackages;
     services.flatpak.enable = true;
     environment.systemPackages = with pkgs; [ gnupg gnome-desktop-testing ostree python2 ];