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?

nixos/modules/services/misc/nix-daemon.nix

@@ -281,7 +281,9 @@ in
       { path = [ nix pkgs.openssl pkgs.utillinux pkgs.openssh ]
           ++ optionals cfg.distributedBuilds [ pkgs.gzip ];
 
-        environment = cfg.envVars // { CURL_CA_BUNDLE = "/etc/ssl/certs/ca-bundle.crt"; };
+        environment = cfg.envVars
+          // { CURL_CA_BUNDLE = "/etc/ssl/certs/ca-bundle.crt"; }
+          // config.networking.proxy.envVars;
 
         serviceConfig =
           { Nice = cfg.daemonNiceLevel;