Bug: possible build race conditions

[drewbo]

It is possible to have non-deterministic builds when two commits are made in quick succession on the same branch. This could potentially be prevented by through our new queue management tools (e.g. somehow only allowing a single build per branch, enforcing order, waiting for completion, cancelling running builds, etc)

Notes

• https://gsa-tts.slack.com/archives/G7EFTFW03/p1659650052463369Additional details, technical information, resource links, and implementation notes
• https://github.com/cloud-gov/private/issues/1200

Acceptance Criteria

• Old builds can't overwrite newer builds