Ensure nix-daemon sees the proxy defined options

(Not sure about this one)

Here are the incomplete and a little contradictory discussions about
it (or at least my interpretations of them):

- https://github.com/NixOS/nixpkgs/pull/5058#discussion_r20830855
- https://github.com/NixOS/nixpkgs/pull/5058#discussion_r21043552

cc @edolstra @wmertens

What did I get wrong?