ci: Look for matching ref in user remote for branch pipeline

Same reasoning than gnome-shell@dbc9ebc6abc8 applies, it may be
useful to run a pipeline with matching branches in the other
project, without necessarily opening a merge request.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3153>

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

@@ -41,8 +41,9 @@ if [ "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
 fi
 
 if [ -z "$gnome_shell_target" ]; then
+  ref_remote=${CI_PROJECT_URL//mutter/gnome-shell}
   echo -n Looking for $CI_COMMIT_REF_NAME on remote ...
-  if fetch origin $CI_COMMIT_REF_NAME; then
+  if fetch $ref_remote $CI_COMMIT_REF_NAME; then
     echo \ found
     gnome_shell_target=FETCH_HEAD
   else