Modify workspace image push logic

Issue - Build golden container image to be used by defa... (gitlab#474984 - closed)

What does this MR do and why?

https://gitlab.slack.com/archives/C03KE0L9NC9/p1736237018273649

This is a followup issue for Workspace Default Image. We are trying to achieve 2 thing here:

• workspace default image manual job: insert the timestamp at the beginning of the image tag & push to registry only on the merge commit (not on MR)
• workspace default image auto job: insert latest at the beginning of the image tag & push to registry all the time when the job runs

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.