impure.nix: fix handling of localSystem

If we passed a localSystem in, we don’t want the current system to
override it. Now we check for localSystem first to avoid getting
"mixed" localSystem values from commands like this:

  nix-build --arg localSystem '{config="x86_64-unknown-linux-musl";}' -A hello

Which would eventually evaluate localSystem to this:

{
  config = "x86_64-unknown-linux-musl";
  system = "x86_64-darwin";
}

& Nix would not be able to run it correctly.

Fixes #41599

/cc @Ericson2314

pkgs/top-level/impure.nix

@@ -83,5 +83,6 @@ import ./. (builtins.removeAttrs args [ "system" "platform" ] // {
   inherit config overlays crossSystem;
   # Fallback: Assume we are building packages on the current (build, in GNU
   # Autotools parlance) system.
-  localSystem = { system = builtins.currentSystem; } // localSystem;
+  localSystem = (if args ? localSystem then {}
+                 else { system = builtins.currentSystem; }) // localSystem;
 })