Automatic update of `spack-packages` definition

[schnellerhase]

Should we automate the sync between spack/spack-packages and the definitions here? There is no ground truth, both packages might be altered independently and tracking consistently is tricky. Ref. https://github.com/marketplace/actions/files-sync.

[jhale]

It might be sensible to do this for a branch(es) of our fork of spack/spack-packages to ease making PRs. My preference is that these are the preferred source, given that the upstream ones are essentially untested (as we found out!).