nixos/activation: use eval-config's system argument for nesting

This avoids a possible surprise if the user is using `nixpkgs.system`
and `nesting.children`. `nesting.children` is expected to ignore all
parent configuration so we shouldn't propagate the user-facing option
`nixpkgs.system`. To avoid doing so, we introduce a new internal
option for holding the value passed to eval-config.nix, and use that
when recursing for nesting.

nixos/lib/eval-config.nix

@@ -41,6 +41,12 @@ let
       # default to the argument. That way this new default could propagate all
       # they way through, but has the last priority behind everything else.
       nixpkgs.system = lib.mkDefault system;
+
+      # Stash the value of the `system` argument. When using `nesting.children`
+      # we want to have the same default value behavior (immediately above)
+      # without any interference from the user's configuration.
+      nixpkgs.initialSystem = system;
+
       _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
     };
   };

nixos/modules/misc/nixpkgs.nix

@@ -216,6 +216,14 @@ in
         Ignored when <code>nixpkgs.pkgs</code> is set.
       '';
     };
+
+    initialSystem = mkOption {
+      type = types.str;
+      internal = true;
+      description = ''
+        Preserved value of <literal>system</literal> passed to <literal>eval-config.nix</literal>.
+      '';
+    };
   };
 
   config = {

nixos/modules/system/activation/top-level.nix

@@ -15,7 +15,7 @@ let
     map (childConfig:
       (import ../../../lib/eval-config.nix {
         inherit baseModules;
-        system = config.nixpkgs.system;
+        system = config.nixpkgs.initialSystem;
         modules =
            (optionals inheritParent modules)
         ++ [ ./no-clone.nix ]