nixos/activation: propagate system to nested configurations

The current behavior lets `system` default to
`builtins.currentSystem`. The system value specified to
`eval-config.nix` has very low precedence, so this should compose
properly.

Fixes #80806

(cherry picked from commit b83164a049)

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

@@ -15,6 +15,7 @@ let
     map (childConfig:
       (import ../../../lib/eval-config.nix {
         inherit baseModules;
+        system = config.nixpkgs.system;
         modules =
            (optionals inheritParent modules)
         ++ [ ./no-clone.nix ]