Makes you wonder why the whole protocol isn't Bittorrent with users seeding their builds to others.
Either don't store your secrets this way or don't use Nix. Please.
/nix/store isn't world-readable. It's machine-readable, and most machines are single-user. I'll explain the caveats, but I can't tell them never to put secrets there; the predictable outcome then is they won't try nixos.
This information could be useful to an attacker.