GitHub workflows to run CI for downstream repos.

Motivation is to have the dependency tree all in once place.

Split into trees for self-hosted runners and HPC.

Contains a build matrix of different platforms and compilers.

When calling downstream CI, specify the ref for each package you need to test with, if it differs from the default value (usually develop). The triggerring repository always needs to pass the input for itself.

See the inputs section and https://docs.github.com/en/actions/using-workflows/reusing-workflows#calling-a-reusable-workflow. Alternatively see one of the repositories which is already in the downstream CI.

Defines dependencies for each package to allow efficient caching. It's used to create the cache key by build-package and build-package-hpc to allow efficient caching.