Expose `/etc/nsswitch.conf` and `/etc/protocols` etc to sandboxed nix builds?

Right now, the following derivation fails in the nix sandbox:

```
with (import <nixpkgs> { config = {}; });

stdenv.mkDerivation {
  name = "networking";
  buildCommand = ''
    ${strace}/bin/strace getent protocols tcp > $out
  '';
}
```

This means that even packages that run their own server on localhost for testing fail, because `getprotobyname` fails. 