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.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/811


(cherry picked from commit fd5989e99a)

.gitlab-ci/checkout-mutter.sh

@@ -1,6 +1,5 @@
 #!/usr/bin/bash
 
-shell_branch=$(git describe --contains --all HEAD)
 mutter_target=
 
 git clone https://gitlab.gnome.org/GNOME/mutter.git
@@ -26,8 +25,7 @@ if [ "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
 fi
 
 if [ -z "$mutter_target" ]; then
-  mutter_target=$(git branch -r -l origin/$shell_branch)
-  mutter_target=${mutter_target:-$(git branch -r -l ${shell_branch#remotes/})}
+  mutter_target=$(git branch -r -l origin/$CI_COMMIT_REF_NAME)
   mutter_target=${mutter_target:-origin/master}
   echo Using $mutter_target instead
 fi