ci: Add dist job

So far, releases are done locally by invoking `meson dist`.

We can do better and leverage the existing CI infrastructure, to get
to the following release workflow:

 - bump version in meson.build, update NEWS etc.
 - open merge request for the release
 - merge when the pipeline (including dist check) succeeds
 - tag the release
 - wait for the tag pipeline to spit out the tarball artifact

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1980>