Copy system channel to the remote host

[adisbladis]

It would be very helpful if we copied the current nixpkgs ${pkgs.path} to the remote so nix-shell & friends actually pull updated packages.

[vikanezrimaya]

aren't channels in NixOS imperative? with the eventual move to flakes I think channels will be obsolete, since e.g. nix shell nixpkgs#hello will always pull from latest unstable or whatever is defined in the flake registry (which can be defined declaratively).

Additionally, users might override the channels for themselves, forget to update and pull outdated stuff forever.