Allow CI/CD mirroring to handle GitHub forks
When using CI/CD for External Projects, if the external project is forked, and a merge request is opened from the fork to upstream project, GitLab CI does not run CI pipelines. We should add support for forking workflows when using external projects with GitLab CI.
- Fetch the source branch of the merge request from the fork, the mirror on GitLab will not have it
We need to consider the security of variables, but GitLab CI should have considered this already for fork networks on GitLab.