ci: install deps from committed lockfiles when present

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

.github/workflows/buildcache.yml

@@ -50,6 +50,21 @@ jobs:
           spack -e fairmq mirror set --oci-username ${{ github.actor }} --oci-password-variable GITHUB_TOKEN ghcr-buildcache
           spack -e fairmq buildcache push --unsigned ghcr-buildcache
 
+      - name: Export lockfile
+        if: ${{ !cancelled() }}
+        shell: spack-bash {0}
+        run: |
+          mkdir -p test/ci/locks
+          cp "$(spack location -e fairmq)/spack.lock" \
+            "test/ci/locks/${{ matrix.env }}-gcc${{ matrix.gcc }}.lock"
+
+      - name: Upload lockfile
+        if: ${{ !cancelled() }}
+        uses: actions/upload-artifact@v4
+        with:
+          name: lock-${{ matrix.env }}-gcc${{ matrix.gcc }}
+          path: test/ci/locks/${{ matrix.env }}-gcc${{ matrix.gcc }}.lock
+
   update-index:
     if: github.repository == 'FairRootGroup/FairMQ' && !cancelled()
     needs: build