nixos/nixpkgs: make config.nixpkgs.{localSystem,crossSystem,buildPlatform,hostPlatform} write only

The description for options.nixpkgs.system already hints at this:

  Neither ${opt.system} nor any other option in nixpkgs.* is meant
  to be read by modules and configurations.
  Use pkgs.stdenv.hostPlatform instead.

We can support this goal by not elaborating the systems anymore, forcing
users to go via pkgs.stdenv.

This will prevent problems when making the top-level package sets
composable in the next commit. For this to work, you should pass a fully
elaborated system to nixpkgs' localSystem or crossSystem options.

nixos/modules/virtualisation/nixos-containers.nix

@@ -506,8 +506,12 @@ in
                         config = {
                           nixpkgs =
                             if options.nixpkgs?hostPlatform
-                            then { inherit (host.pkgs.stdenv) hostPlatform; }
-                            else { localSystem = host.pkgs.stdenv.hostPlatform; }
+                            then {
+                              hostPlatform =
+                                if host.options.nixpkgs.hostPlatform.isDefined
+                                then host.config.nixpkgs.hostPlatform
+                                else lib.defaultTo host.config.nixpkgs.localSystem host.config.nixpkgs.crossSystem;
+                            } else { localSystem = lib.defaultTo host.config.nixpkgs.localSystem host.config.nixpkgs.crossSystem; }
                           ;
                           boot.isContainer = true;
                           networking.hostName = mkDefault name;