Lookup job id when looking for artifacts from upstream pipeline

Unfortunately the interface [1] we were using previously will not provide an artifact until a pipeline has successfully finished on the requested branch, thus rendering it useless for use from jobs triggered from MRs.

This partially reverts commit 00db7a0d.

[1] https://docs.gitlab.com/ee/api/job_artifacts.html#download-a-single-artifact-file-from-specific-tag-or-branch