Motivation for this change

Gosu tool was removed with the go package rewrite. The package is mainly used in Docker images, but not limited to it.

It's used in a popular series of blog post by Luca Bruno aka @lethalman .
http://lethalman.blogspot.si/2016/04/cheap-docker-images-with-nix_15.html

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
  • Linux
• 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.