ci: Add release job
It is more or less copied from mutter. One job is for running meson dist
a bit more often, and the other is for uploading tarballs as
artifacts on tags.
Ran it with the commit env var hard coded to 48.alpha here: https://gitlab.gnome.org/jadahl/xdg-desktop-portal-gnome/-/commit/a2253fa0d7853fa814bd1c62c7f6558f542d4ef0/pipelines?ref=wip%2Fci-tarball.