Duplicate CI pipelines for renovate-runner self-updates

Hi,

I noticed our self-hosted renovate-runner based on this repository creating two pipelines per renovate branch/MR in the renovate-runner project and I think I tracked it down to this line in .gitlab-ci.yml: https://gitlab.com/renovate-bot/renovate-runner/-/blob/main/.gitlab-ci.yml?ref_type=heads#L20

The CI_COMMIT_BRANCH variable is "Not available in merge request pipelines or tag pipelines." ¹, I think the CI_MERGE_REQUEST_SOURCE_BRANCH_NAME is the correct variable to use here instead?

 workflow:
   rules:
     # Don't build push pipeline when open MR and not renovate branch
     - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS && $CI_COMMIT_BRANCH !~ /^renovate\//
       when: never
     # Don't build MR pipeline when renovate branch
-    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_COMMIT_BRANCH =~ /^renovate\//
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME =~ /^renovate\//
       when: never
     # Don't build tags
     - if: $CI_COMMIT_TAG
       when: never
     - when: always

1. https://docs.gitlab.com/ee/ci/variables/predefined_variables.html#:~:text=Not%20available%20in%20merge%20request%20pipelines%20or%20tag%20pipelines. ↩