buildEnv: break with a proper error if one path is actually a file

I noticed by creating `buildEnv` where I accidentally put a derivation
from `pkgs.writeText` into `paths` and got a broken build with the
following misleading error message:

```
Use of uninitialized value $stat1 in numeric ne (!=) at /nix/store/9g4wc31j7a2xp22xpgwr0qssfxahxdzl-builder.pl line 74.
Use of uninitialized value $stat1 in bitwise and (&) at /nix/store/9g4wc31j7a2xp22xpgwr0qssfxahxdzl-builder.pl line 75.
different permissions in `' and `/nix/store/0vy5ss91laxvwkyvrbld5hv27i88qk5w-noise': 0000 <-> 0444 at /nix/store/9g4wc31j7a2xp22xpgwr0qssfxahxdzl-builder.pl line 75.
```

It can be reproduced with an expression like this:

``` nix
{ pkgs ? import <nixpkgs> { } }:
let
  file = pkgs.writeText "test" ''
    content
  '';
in
  pkgs.buildEnv {
    name = "test-env";
    paths = [ /* ... */ file ];
  }
```