Nix and shebangs don't work together

Given a script of:

``` haskell
#!/usr/bin/env stack
-- stack --nix --resolver lts-5.9 runghc

main = return ()
```

We get:

```
File does not exist or is not a regular file `--extra-lib-dirs=/nix/store/n962p1idgzh07wvhxam5r6icv894kj61-ghc-7.10.3/lib'
```
