Propagate PIP index URL

What does this MR do?

Propagate PIP_INDEX_URL and PIP_EXTRA_INDEX_URL to the Dependency Scanning analyzer gemnasium-python to allow for custom PyPI registries.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

/cc @janmasarik

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/12810

This replaces https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13769