nixos: Add all of root's channels to NIX_PATH.

This is very useful if you want to distribute channels (and thus
expressions as well) in a similar fashion to Debians APT sources (or
PPAs or whatnot).

So, for example if you have a channel with some additional functions
or packages, you simply add that channel with:

sudo nix-channel --add https://example.com/my-nifty-channel

 foo

And you can access that channel using <foo>, for example in your
configuration.nix:

{
  imports = [ <foo/modules/shiny-little-module> ];
  environment.systemPackages = with import <foo/pkgs> {}; [ bar blah ];
  services.udev.extraRules = import <foo/lib/udev/mkrule.nix> {
    kernel = "eth*";
    attr.address = "00:1D:60:B9:6D:4F";
    name = "my_fast_network_card";
  };
}

Within nixpkgs, we shouldn't have <nixos> used anywhere anymore, so we
shouldn't get into conflicts.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>