Simplify variable symlink setup for users

[ocaisa]

We have an increasing number of variable symlinks which I suspect is going to get longer. It would be nice to have a helper script to set these up for people. I am thinking:

• Set up the basic variant symlink for host_injections
• Have the script create a default structure under /cvmfs/software.eessi.io/host_injections that covers
  • EESSI version-specific installations (/cvmfs/software.eessi.io/host_injections/2023.06, etc., for all EESSI versions under /cvmfs/software.eessi.io/versions)
  • GPU drivers under /cvmfs/software.eessi.io/host_injections/drivers
• Report back how variant symlinks should be set, and how to verify them
• Currently in our glibc we hard code some host paths, we should switch these to variant symlinks and do auto-discovery in the script
  • Current hard-coded paths can be replaced as default targets of the symlinks
  • Symlinks are only set if they are actually needed