I opened an issue describing a well-bounded potential installer fix/improvement.
In short, it would be nice to correct for something that backslid when we replaced rsync with cp and chmod: idempotently copying the initial store seed files into place.
If you already know the answer, it should be a small change plus a little VM testing. You can also see https://github.com/NixOS/nix/pull/6652 for detail on how to make the CI process generate testable installers.