nixos-rebuild: Conditionally update channels.

This partially reverts commit 3a4fd0bf

.

Addresses another concern by @edolstra that users might not want to
update *all* channels. We're now reverting to the old behaviour but
after updating the "nixos" channel, we just check whether the channel
ships with a file called ".update-on-nixos-rebuild" and if it exists, we
update that channel as well.

Other channels than these are not touched anymore.

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