Geant4 libraries not seen during runtime in downstream projects

@clemenci @gcorti @dpopov

This issue popped up when running simulations with Gauss-on-Gaussino with the new CMake.

The dependency is: GaussinoExtLibs (here we pick up Geant4) -> Gaussino working ok -> Gauss mising shared libraries during runtime

The error I get is:

====================================================================================================================================
                                                   Welcome to Gauss version 60.0
                                          running on mgaussino.cern.ch on Tue Aug 30 11:53:50 2022
====================================================================================================================================
WARNING: cannot load libGiGaMT.so for factory GiGaMT
WARNING: libG4run.so: cannot open shared object file: No such file or directory
ServiceManager      FATAL No Service factory for GiGaMT available.
ApplicationMgr      ERROR decodeExtSvcNameList: Cannot create service GiGaMT/GiGaMT
ApplicationMgr      ERROR Failure during external service association
terminate called after throwing an instance of 'GaudiException'

The workaround for this is in Gauss!885 (merged) where I explicitly prepend the LD_LIBRARY_PATH again.