ci: Try falling back to stable branch before HEAD

For non-MR pipelines, we check for a matching branch or tag name in mutter. As we use the same naming policy for stable branches and branch at about the same time, this works generally fine for branches.

However for tags, it is less unlikely that there is no matching tag in mutter, in particularly late in a stable cycle when most changes are translation updates (which are much rare in mutter than gnome-shell).

Before falling back to main (which is doomed to fail in that case), try to guess the correct stable branch based on the tag name.