ci: use fallback key
MRs especially those from renovate bot take a long time because they create fresh branches with no cache associated
This MR introduces the use of a fallback key, when no primary cache is found we resort to following keys:
- MR Target branch
- Default branch
These keys don't support slugs as of yet but hopefully this gets added soon: https://gitlab.com/gitlab-org/gitlab/-/issues/479896
Edited by Jasper Vaneessen