CI: build of dependents required when changes are made in spack_repo/access/nri/build_systems/

[harshula]

It appears there was no CI build of the dependents of um_base.py when it was modified in a PR:

• Changes to UM build system and CABLE package to build AM3 with CABLE library #460
• Cable is dep only when +cable #466

[CodeGat]

I've investigated a quick solution which treats build_systems similarly to packages, in which we build manifests with packages that use those base classes (um, gcom), just by adding them to .github/build-ci/manifests/um_base/*. There would be a bit of duplication for manifests like that, though. See https://github.com/ACCESS-NRI/access-spack-packages/tree/467-ci-for-build-systems for a PoC.

@harshula noted that we should try to eliminate duplication as much as we can. So, we would need to set up some kind of special mapping between build_systems packages and particular manifests (whether they be sourced from CRs or access-spack-packages itself). This will require a bit more thought, so I'll leave it until other work (like ACCESS-NRI/build-cd#404) is completed