ci: Fix checking out mutter on stable branches

For stable branches, we currently only check out the correct mutter branch for merge requests. For the regular pipeline, our code to determine the current shell branch fails because CI runs on a temporary "pipeline/12345" branch that doesn't exist for mutter.

Switching to the correct gitlab environment variable fixes that.

I tested this under my namespace and it worked there, so here's hoping for a green checkmark after cherry-picking to gnome-3-34 :-)