Unify NixOS and Nixpkgs channel structure

This is primarily to ensure that

  -I nixpkgs=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz

and

  -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz

and

  -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz

behave consistently.

It also allows installing packages via "nix-env -iA nixos.<pkg>"
rather than "nixos.pkgs.<pkg>". It would be even better to allow
"nixpkgs.<pkg>", but that requires a change to nix-channel.

Fixes #7659.

nixos/lib/build-vms.nix

@@ -1,6 +1,6 @@
 { system, minimal ? false }:
 
-let pkgs = import ./nixpkgs.nix { config = {}; inherit system; }; in
+let pkgs = import ../.. { config = {}; inherit system; }; in
 
 with pkgs.lib;
 with import ../lib/qemu-flags.nix;