Geant4 Fixes, main branch (2023.09.06.)

Unfortunately I was not careful enough with !1059 (merged). 😦 As the version of Geant4 that we currently use, does not actually support being built in C++20 mode. 😦 (Later versions do.)

https://gitlab.cern.ch/atlas-simulation-team/geant4/-/blob/v10.6.3.8/cmake/Modules/G4BuildSettings.cmake#L156-171

But since I really wanted to build G4 itself in C++20 mode as well, I tested how hard it would be to force it to do so, and it wasn't too hard. 😌 I just needed to patch the CMake configuration of Geant4 for it, the code was building fine in C++20 mode.

As long as I was doing this, I also improved a bit on how the !$%^& symbolic link would be handled in the G4 installation. Since our current setup for removing it as part of the post-install step is pretty fragile. (It doesn't work when not using -DCTEST_USE_LAUNCHERS=TRUE for instance.) So I now rather opted for patching the G4 code not to create the symbolic link in the first place.

Pinging @elmsheus, @mbandier and @jchapman for info. 😉