Motivation for this change

The comment in this file about why nox is here
was "Needed by travis-ci to test PRs", and we
for sure don't use travis-ci anymore.
Instead we're making nixpkgs-review a deliverable
because it's included in the PR template as a tool
to use. We also swapped out nox for nixpkgs-review
in the PR template, so I believe this makes sense
here because it was changed.

cc @Mic92, because software you wrote will be a release deliverable.

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.