Reusing concretization between the weekly buildcache (fresh) and weekday CI
(reuse) can drift if runner externals change, causing avoidable cache misses.
- setup-deps installs from test/ci/locks/<env>-gcc<N>.lock when it exists,
skipping concretization for byte-identical hashes; falls back to the spec
yaml otherwise
- buildcache exports each env's spack.lock as a downloadable artifact so the
lockfiles can be regenerated on the ubuntu-24.04 runner and committed
- document the manual regeneration flow in test/ci/locks/README.md
- rename mirror to ghcr-buildcache
- find system compiler before building gcc
- separate update-index job to avoid race condition
- always attempt push even on partial failure
BREAKING CHANGE
Due to a lack of users, we remove the experimental code. The
latest implementation can be found in release v1.4.56. This does
not mean it will never be picked up again, but for now there are
no plans.
