ci: Skip jobs already passed in MR

This project's GitLab repository only permits fast forward merges,
meaning that the SHA1 of the target branch (e.g. main) is always that
of the last merged merge request. Save CI resources and minimize delays
to mirroring to the public GitLab, by not rerunning tests on identical
SHA1.

`.base-job-rules` in `deveoper-tools` defines the actual rules to run
the jobs.

For pipeline to run, there must be atleast one job other than jobs in
stages `.pre` and `.post`. Currently main branch runs `sync-public` in
`.post`. Move the job to explicit stage so that GitLab CI works on
`main` branch.

Signed-off-by: Saheer Babu <saheer.babu@arm.com>