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 ];
}
```
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