Make sure we're targeting the right branch in the canonical project when retrieving test_metadata_job_id

What does this MR do?

Make sure we're targeting the right branch in the canonical project.

Forks might not be sharing with the same $CI_DEFAULT_BRANCH with the canonical project.

Fixes gitlab-jh/gitlab#29 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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