flatpak-nightly: Disable cloning GStreamer submodules

It seems that now GStreamer ships its testsuite (which we do not need) as a submodule. Do not try to clone it.

pkgs/flatpak/testing/gstreamer.json

@@ -66,7 +66,8 @@
         {
             "type": "git",
             "url": "https://gitlab.freedesktop.org/gstreamer/gstreamer.git",
-            "branch": "main"
+            "branch": "main",
+            "disable-submodules": true
         }
     ]
 }