Describe the bug
Looking at the binary cache directory on the macOS CI system, I see that the directory is empty. Need to check what is going on, and wether that is related to the update from the authoritative spack repository (which removed the "ignore error while creating multiple build caches at once" capability).

To Reproduce
n/a

Expected behavior
Build cache is used on macOS CI, builds are fast.

System:
macOS CI system

Additional context
I submitted as PR to mainline spack that will fix the problem for us: spack/spack#38496

Once that PR is merged, it is time to bring in the collective updates from spack to our fork, which will close this issue.