CI: Run the distcheck job for merge request pipelines
CI: Run the distcheck job for merge request pipelines

Don't want merge requests like the last one where distcheck fails, but
we only find out later!

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/707>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg