nix-shell: Interpret filenames relative to the #!-script

So you can have a script like:

  #! /usr/bin/env nix-shell
  #! nix-shell script.nix -i python

  import prettytable

  x = prettytable.PrettyTable(["Foo", "Bar"])
  for i in range(1, 10): x.add_row([i, i**2])
  print x

with a ‘script.nix’ in the same directory:

  with import <nixpkgs> {};

  runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } ""

(Of course, in this particular case, using the ‘-p’ flag is more
convenient.)