When it comes to copying kernels, test /nix/store instead of /nix

Salut !

Congratulations for your work ! I've tried it on my laptop and it works great, except that its `/nix/store` is on a separate partition instead of its `/nix`. I honestly don't remember where this idea came from, whether I made it up or found it in some piece of documentation, but I don't think the fix I suggest would break anything.

Regards,
Louis