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/ci.yml

@@ -13,7 +13,7 @@ jobs:
   build:
     if: github.repository == 'FairRootGroup/FairMQ'
     name: ${{ matrix.env }}-gcc-${{ matrix.gcc }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix:
@@ -56,7 +56,7 @@ jobs:
   sanitizers:
     if: github.repository == 'FairRootGroup/FairMQ'
     name: ${{ matrix.sanitizer.name }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix:
@@ -102,7 +102,7 @@ jobs:
   static-analysis:
     if: github.repository == 'FairRootGroup/FairMQ'
     name: static-analysis
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
 
     steps:
       - uses: actions/checkout@v6