Revert "Merge branch 'nix-run' of git://github.com/rickynils/nixpkgs"

nix-run's functionality is subsumed by nix-shell -p.

This reverts commit 3cc2b243c7f2d906ce77968e75fd84a3f65a09e0, reversing
changes made to 4d5d6aed29a0ee10a471fd870fa2fdb61408e4eb.