Introduce FIND_CHANGES_* variables

What does this MR do and why?

Introduce FIND_CHANGES_* variables

This should make the intention more clear and avoid messing up

This is a follow up from !148560 (merged)

• Public detect-tests in as-if-foss pipeline still works: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/6685574767
• Security detect-tests in as-if-foss pipeline now works: https://gitlab.com/gitlab-org/security/gitlab-foss/-/jobs/6685653830

This is tested in https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/3982

See https://gitlab.com/gitlab-org/gitlab/-/issues/456936