This should help resolve the "No binary found when cache-only was specified"
errors we've recently seen in our GitLab CI pipelines.
example failing job here:
https://gitlab.spack.io/spack/spack/-/jobs/15570931#L370
This error is caused when a generate job finds a spec in the local root
binary mirror, and that spec does not yet exist in the stack-specific mirror.
The fix here is to instead locally cache the stack-specific mirrors and only
use the root-level mirror for public use.
f5ff63e68d