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/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 ]