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