G4PATH Improvement, master branch (2021.02.03.)

Made it possible to change the default path that G4 data files would be picked up from. This is necessary for non-standard nightlies that don't use RPMs for installing themselves on CVMFS, as discussed in ATLINFR-3946.

Once this is in, the "LCG nightlies" will need to set the following environment variable during their build:

export G4PATH=/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc8-opt/Geant4

Pinging @jchapman, @elmsheus and @simmons.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST-CC7 #446]

merged

mentioned in commit 8368f61b