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
Use JSON output and jq to select the newest installed gcc version
when multiple versions match, avoiding conflicts between system and
spack-installed compilers.
