Fallback to using 'CI_COMMIT_SHA' when pipeline is 'detached'

What does this MR do?

This is a follow-up of !26130 (merged).

When a pipeline runs in detached mode (e.g. when the MR is a WIP), CI_MERGE_REQUEST_SOURCE_BRANCH_SHA (and CI_MERGE_REQUEST_TARGET_BRANCH_SHA) is empty, as can be seen in https://gitlab.com/gitlab-org/gitlab/-/jobs/455921920 (compared to https://gitlab.com/gitlab-org/gitlab/-/jobs/455909447 which ran in merged_result mode).

This MR makes the downstream pipelines for CNG and omnibus-gitlab fallback to pass CI_COMMIT_SHA instead of CI_MERGE_REQUEST_SOURCE_BRANCH_SHA if it's empty.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content