Use 'rules' in '.gitlab/ci/cache-repo.gitlab-ci.yml'

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab/ci/cache-repo.gitlab-ci.yml

+# Make sure to update all the similar conditions in other CI config files if you modify these conditions
+.if-cache-credentials-schedule: &if-cache-credentials-schedule
+  if: '$CI_REPO_CACHE_CREDENTIALS && $CI_PIPELINE_SOURCE == "schedule"'
+
 # Builds a cached .tar.gz of the master branch with full history and
 # uploads it to Google Cloud Storage. This archive is downloaded by a
 # script defined by a CI/CD variable named CI_PRE_CLONE_SCRIPT. This has
@@ -33,8 +37,6 @@ cache-repo:
     - tar cf $TAR_FILENAME .
     - gzip $TAR_FILENAME
     - gsutil cp $TAR_FILENAME.gz gs://gitlab-ci-git-repo-cache/project-$CI_PROJECT_ID/gitlab-master.tar.gz
-  only:
-    variables:
-      - $CI_REPO_CACHE_CREDENTIALS
-    refs:
-      - schedules
+  rules:
+    - <<: *if-cache-credentials-schedule
+      when: on_success