ci: cache gcc as a buildcache node instead of committed lockfiles

Committed lockfiles pinned gcc as a host-path external (from spack compiler
find), which is not portable across runners and broke CI. Cache the gcc
compiler itself as a buildcache node instead, so CI pulls it (~1 min) rather
than building it from source (~1 h).

- push the freshly-built gcc node in setup-deps BEFORE spack compiler find
  (which marks it external and excludes it from buildcache push), gated behind
  a push-gcc input used only by the buildcache workflow
- drop the committed-lockfile approach: remove test/ci/locks, the lockfile
  install path in setup-deps, and the lockfile export in the buildcache workflow
- drop the ignored ref input from setup-spack (v3 renamed it to spack_ref)

.github/workflows/buildcache.yml

@@ -40,6 +40,8 @@ jobs:
           gcc: ${{ matrix.gcc }}
           env: ${{ matrix.env }}
           fresh: 'true'
+          push-gcc: 'true'
+          push-token: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Push to buildcache
         if: ${{ !cancelled() }}
@@ -50,21 +52,6 @@ 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
@@ -79,7 +66,6 @@ jobs:
       - name: Setup spack
         uses: spack/setup-spack@v3
         with:
-          ref: v1.1.0
           color: true
 
       - name: Update buildcache index