Allow GIT_REPO_URL to be overridden for self managed patch releases

What does this MR do?

This addition allows us to configure the omnibus package built for self-managed to change which repo we leverage for the Git software if ever needed. This allows us to build packages by pulling the git source from https://gitlab.com/gitlab-org/security/git.git if required, instead of https://gitlab.com/gitlab-org/git.git.

The variables defined here will only be used for auto-deploy pipelines or self-managed patch releases, so we can add the variables as project variables in https://dev.gitlab.org/gitlab/omnibus-gitlab without affecting other Omnibus pipelines.

This is similar to !6509 (merged).

Related issues

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
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened