RMG-Core: Fix GNUInstallDirs usage

[OPNA2608]

One may not manually prepend CMAKE_INSTALL_*DIR variables with CMAKE_INSTALL_PREFIX, because CMAKE_INSTALL_*DIR variables may already be absolute paths - inside or outside of CMAKE_INSTALL_PREFIX.

To get a guaranteed-absolute version of them, CMAKE_INSTALL_FULL_*DIR must be used.

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Spotted in NixOS/nixpkgs#284124 (comment):

terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error' what():  filesystem error: recursive directory iterator cannot open directory: No such file or directory [/nix/store/zv4gsdpzdkpb3aynj3h9agk8wzzyrzj9-rmg-0.5.6//nix/store/zv4gsdpzdkpb3aynj3h9agk8wzzyrzj9-rmg-0.5.6/lib/RMG//Core] Aborted (core dumped)

I have left CORE_INSTALL_PREFIX incase you still need it somewhere in the future, though it will currently be unused.

[Rosalie241]

Thank you for the patch, I apologize for the breakage and not reading the documentation correctly! There are always so many subtle things to watch out for ❤️