Switch LCG and AthenaExternals setup
This is an attempt to fix the warnings like
+Warning in <TInterpreter::ReadRootmapFile>: While processing /cvmfs/atlas-nightlies.cern.ch/repo/sw/main--ACTS_Athena_x86_64-el9-gcc13-opt/2024-10-15T2101/Athena/25.0.20/InstallArea/x86_64-el9-gcc13-opt/lib/Athena.rootmap, class HepMC3::FourVector was found to be associated to libGeneratorObjectsDict.so although it is already associated to libHepMC3rootIO.so
+Warning in <TInterpreter::ReadRootmapFile>: While processing /cvmfs/atlas-nightlies.cern.ch/repo/sw/main--ACTS_Athena_x86_64-el9-gcc13-opt/2024-10-15T2101/Athena/25.0.20/InstallArea/x86_64-el9-gcc13-opt/lib/Athena.rootmap, class HepMC::FourVector was found to be associated to libGeneratorObjectsDict.so although it is already associated to libHepMC3rootIO.so