Update CI config

• Update default LCG version to 103
  • HepMC2 build uses LCG_103_LHCB_7
• Add builds for el9 OS using LCG 104
• Allow switching on/off building against each external (default to all on)
• Test stage: attempt to improve selection of commits to diff in different cases
• Update CVMFS tags as per https://cern.service-now.com/service-portal?id=outage&n=OTG0079356