bug: create_tag pipeline failing

The create_tag stage of the pipeline is continuing to fail: https://repo1.dso.mil/big-bang/apps/developer-tools/bbctl/-/jobs/52533411#L36

The pipeline expects a COMMIT_BRANCH environment variable to be present with which it parses the release branch from. For some reason, this isn't working in the pipelines.

We currently specify:

  variables:
    COMMIT_SHA: $CI_COMMIT_SHA
    COMMIT_BRANCH: $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME

Investigate and fix.