WIP: Propagate, document PIP_DEPENDENCY_PATH

What does this MR do?

Add missing PIP_DEPENDENCY_PATH variable to ENVs to be propagated to ~"dependency scanning" container

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

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/12412