Modify Geant4Config.cmake to set data files env variables for LHCb projects

With Gauss!830 (merged), Gauss needs that the upstream projects (Geant4 in particular) declare the environment variables they need via the <Project>Config.cmake file.

This MR tweaks the original Geant4Config.cmake to make sure the environment variables used to access Geant4 data files are correctly propagated.