ci: Trigger omnibus pipelines against corresponding stable branch

What does this MR do and why?

This was inadvertently removed by !96509 (merged).

This will ensure that e2e:package-end-test-* jobs that are triggered from stable branches trigger downstream gitlab-org/build/omnibus-gitlab-mirror pipelines against the corresponding stable branch instead of always master.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #409059 (closed)