nixos: Put root's channels to the end of NIX_PATH.

My original reason to put it at the beginning of NIX_PATH was to allow
shipping a particular version <nixpkgs> with a channel. But in order to
do that, we can still let the channel expression ship with a custom
version of nixpkgs by something like <channel/nixpkgs> and the builder
of the channel could also rewrite self-references.

So the inconvenience is now shifted towards the maintainer of the
channel rather than the user (which isn't nice, but better err on the
side of the developer rather than on the user), because as @edolstra
pointed out: Having the channels of root at the beginning of NIX_PATH
could have unintended side-effects if there a channel called nixpkgs.

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