nixos/install-grub: Don't run GrubFs(/nix/store) if copyKernels is true

@wkennington This was the quickest solution to disable running `GrubFs` on `/nix/store` when `copyKernels` is true, but I haven't digged through the Perl script enough to say if it is a good or bad solution. Feel free suggesting something else or submit another PR.

Some context: `GrubFs` fails when `/nix/store` is on NFS or CIFS, but in those cases `copyKernels` will be true anyway, and we shouldn't need to find a grub device. So we can simply skip running it.