Release tarball is .xz but we currently use a snapshot from gitlab, so just mirror the .bz2 instead of changing all URLs. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7221>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5775>
And allow fallback to it. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4447>