Make nix packages additive between stack.yaml and ~/.stack/config.yaml

My understanding is that the nix packages from the local stack.yaml override the nix packages from ~/.stack/config.yaml. That means that if I need to add a project-specific nix package to stack.yaml, I also need to copy all of the nix packages from ~/.stack/config.yaml.

Is this an accurate understanding of the current behavior? If so, I would find it much more convenient if stack instead added the package lists together. Is this as easy as changing nixMonoidPackages in NixOptsMonoid from `First [Text]` to `[Text]`?