As kinfocenter's implementation consists of only KCMs, which are
actually just displayed using systemsettings5' KCM shell, the
kinfocenter binary is just a symlink to systemsettings5.

In traditional FHS environments, having a relative symlink to the
systemsettings5 binary within the same bin/ directory works just
fine, whereas this is broken on NixOS where systemsettings5 resides in
a completely different bin/ directory of the corresponding store item.

Usually CMake's find_package should be able to locate the
systemsettings5 binary, but fails for reasons unknown to do so on
NixOS - so for now fixing the symlink manually as part of the preFixup
phase will do the job of making kinfocenter work again.

Motivation for this change

Get rid of this message when starting Info Center:

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.