Small installer idempotence fix?

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 for detail on how to make the CI process generate testable installers.