treewide: fix loss of precision in NixOS systems

Prior to this patch:

	$ nix-instantiate --eval -E '
	>   with import ./. {
	>     localSystem.config = "aarch64-unknown-linux-musl";
	>   };
	>   (nixos {}).config.nixpkgs.localSystem.config
	> '
	"aarch64-unknown-linux-gnu"

Because only the system triple was being passed through, the Musl part
of the system specification was lost.  This patch fixes various
occurrences of NixOS evaluation when a Nixpkgs evaluation is already
available, to pass through the full elaborated system attribute set,
to avoid this loss of precision.

nixos/modules/virtualisation/nixos-containers.nix

@@ -284,7 +284,7 @@ let
     DeviceAllow = map (d: "${d.node} ${d.modifier}") cfg.allowedDevices;
   };
 
-  system = config.nixpkgs.localSystem.system;
+  inherit (config.nixpkgs) localSystem;
   kernelVersion = config.boot.kernelPackages.kernel.version;
 
   bindMountOpts = { name, ... }: {
@@ -478,12 +478,12 @@ in
               type = lib.mkOptionType {
                 name = "Toplevel NixOS config";
                 merge = loc: defs: (import "${toString config.nixpkgs}/nixos/lib/eval-config.nix" {
-                  inherit system;
                   modules =
                     let
                       extraConfig = {
                         _file = "module at ${__curPos.file}:${toString __curPos.line}";
                         config = {
+                          nixpkgs = { inherit localSystem; };
                           boot.isContainer = true;
                           networking.hostName = mkDefault name;
                           networking.useDHCP = false;

nixos/tests/containers-imperative.nix

@@ -18,8 +18,9 @@ import ./make-test-python.nix ({ pkgs, lib, ... }: {
       # container available within the VM, because we don't have network access.
       virtualisation.additionalPaths = let
         emptyContainer = import ../lib/eval-config.nix {
-          inherit (config.nixpkgs.localSystem) system;
           modules = lib.singleton {
+            nixpkgs = { inherit (config.nixpkgs) localSystem; };
+
             containers.foo.config = {
               system.stateVersion = "18.03";
             };

pkgs/top-level/all-packages.nix

@@ -34253,11 +34253,11 @@ with pkgs;
     configuration:
       let
         c = import (path + "/nixos/lib/eval-config.nix") {
-              inherit (stdenv.hostPlatform) system;
               modules =
                 [(
                   { lib, ... }: {
                     config.nixpkgs.pkgs = lib.mkDefault pkgs;
+                    config.nixpkgs.localSystem = lib.mkDefault stdenv.hostPlatform;
                   }
                 )] ++ (
                   if builtins.isList configuration