Prior to this change, man pages from Nixpkgs written using the mdoc(7)
macros would start like this:

NC(1)                   BSD General Commands Manual                   NC(1)

and end like this:

BSD                          December 27, 2018                          BSD

No matter what operating system they were run on.

It's far more accurate to say "Nixpkgs General Commands Manual", so
with this patch we configure groff to do just that. The variable is
called "operating-system", but I think it makes more sense to say
"Nixpkgs" than "NixOS" or something, because packages from Nixpkgs can
run on lots of operating systems, and the important thing is that the
package is from Nixpkgs.

Motivation for this change

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.