Restore VecGeom build and use by Geant4

Following resolution of the stuck track issue reported in ATLASSIM-5739, and identification in VECGEOM-601, it was requested that the VecGeom build and use by Geant4 be reinstated.

This MR implements this, with the following updates to the VecGeom and Geant4 externals:

• vecgeom:
  • Bump version to 1.1.20
  • Update build args for its new CMake scripts
  • Add local patch for consistent tolerance in cones
    • See VecGeom/VecGeom!874 (merged)
    • And https://sft.its.cern.ch/jira/browse/VECGEOM-601
• Geant4:
  • Patch Geant4's CMake shim for VecGeom to adapt to new target name and include path
  • Update FindGeant4.cmake for VecGeom header paths and static library

Pinging @jchapman and @mbandier for their info.