nix-shell: Add --run flag

‘--run’ is like ‘--command’, except that it runs the command in a
non-interactive shell. This is important if you do things like:

  $ nix-shell --command make

Hitting Ctrl-C while make is running drops you into the interactive
Nix shell, which is probably not what you want. So you can now do

  $ nix-shell --run make

instead.