nixos-rebuild: Update all channels of user root.

Should make it even easier to use custom channels, because whenever the
user does a "nixos-rebuild --upgrade", it will also upgrade possibly
used ("used" as in referenced in configuration.nix) channels besides
"nixos". And if you also ship a channel tied to a particular version of
nixpkgs or even remove the "nixos" channels, you won't run into
unexpected situations where the system is not updating your custom
channels.

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