The scheduled pipeline `Sync master to main-jh` should only do the sync but not any other jobs

As part of https://gitlab.com/gitlab-jh/gitlab/-/merge_requests/19, we setup a scheduled pipeline Sync master to main-jh at https://gitlab.com/gitlab-jh/gitlab/-/pipeline_schedules to automatically merge master to main-jh. However we found this pipeline will not only do the merge but also run all other jobs which is actually not necessary because the merge itself will trigger another pipeline to run all the jobs.

We'd better to find a way to let this scheduled pipeline only do the merge.