ci: Fix checkout out gnome-shell on stable branches

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

Switching to the correct gitlab environment variable fixes that.

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

.gitlab-ci/checkout-gnome-shell.sh

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