top-level: Do stdenvOverrides in stage.nix even if crossSystem exists.

Instead, the cross stdenv will patch up the override field -- the complexity
is now confined to the one place it matters.

pkgs/stdenv/cross/default.nix

@@ -24,6 +24,7 @@ in bootStages ++ [
     stdenv = vanillaPackages.stdenv // {
       # Needed elsewhere as a hacky way to pass the target
       cross = crossSystem;
+      overrides = _: _: {};
     };
   })