virtualbox: Remove group check on /nix/store.

This is espacially cruicial when it comes to Nix 1.9, where we even have
a more restrictive /nix/store. In any event, VirtualBox in hardenend
mode doesn't have to check the /nix/store path, because it's read-only
on NixOS systems. So this check would not introduce more security but
more hurdles, thus I'm removing it (of course _only_ for /nix/store).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>