Build KAS from SHA

What does this MR do?

This is a revert of a revert. (Original work !1074 (merged))

As discussed in gitlab-org/cluster-integration/gitlab-agent#300 (closed) this is a second attempt to coordinate all the necessary steps to move KAS on a proper location on dev.

• We change how we download to match existing patterns such that we can download target ref's of the GitLab Agent for Kubernetes
• Changes out the use of the CI_SERVER_* variables in favor of more common CI_API_V4_URL and a token for which to leverage for downloading target source code via the API

Related issues

Related to gitlab-org/cluster-integration/gitlab-agent#300 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated - n/a - this is a change to how the image is built, not to any configuration
• Integration tests added to GitLab QA - n/a - what tests?
• The impact any change in container size has should be evaluated