ci: fetch gcc from buildcache mirror and pin runner image

gcc was built from source (~58 min/job) because the FairMQ buildcache mirror
is only configured inside the env yaml, while gcc is installed before the env
is created.

- register the mirror globally after spack setup so "Install GCC" pulls the
  compiler as a binary
- pin runners to ubuntu-24.04 so the weekly buildcache and weekday CI share an
  image and concretize to matching hashes
- bump setup-spack to v3 to match the update-index job

.github/workflows/buildcache.yml

@@ -18,7 +18,7 @@ jobs:
   build:
     if: github.repository == 'FairRootGroup/FairMQ'
     name: ${{ matrix.env }}-gcc-${{ matrix.gcc }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     permissions:
       packages: write
     strategy:
@@ -52,7 +52,7 @@ jobs:
   update-index:
     if: github.repository == 'FairRootGroup/FairMQ' && !cancelled()
     needs: build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     permissions: