Motivation for this change

sg and newgrp only changes the current user session and should be
available to users even if the "users.mutableUsers" option is set.
These are common, useful commands.

chfn does modify the /etc/passwd GECOS field which is also controlled
by the option "users.users.<name?>.description", so it's less
appropriate to make it available when "users.mutableUsers" is set.

However, because CHFN_RESTRICT in login.defs is never set in current
NixOS the chfn functionality is never available to users anyway and
may as well have its SUID disabled, as only root is able to use it.
This is recommended in the chfn man page in this case.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.