Add AUTO_DEPLOY_COMPILE_ASSETS "feature flag"

What does this MR do?

When present as a CI variable, this will essentially keep the existing behavior, where auto-deploy builds skip the fetch-assets job and force the COMPILE_ASSETS environment variable.

When removed, the build should attempt to use the fetch_assets script and pull the gitlab-assets-ee image tagged with the gitlab-rails SHA.

Related issues

#5805

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
  • Create AUTO_DEPLOY_COMPILE_ASSETS CI variable on dev.gitlab.org
  • Merge this MR
  • Current behavior should be exactly the same: auto-deploy package builds compile assets
  • Delete the CI variable
  • Auto-deploy package builds should run the fetch-assets job and pull the gitlab-assets-ee:[rails SHA] image
• Documentation created/updated
• Tests added
• [-] Integration tests added to GitLab QA
• [-] Equivalent MR/issue for the GitLab Chart opened