virtualbox: Allow /nix/store being world-writable.

We are already checking whether /nix/store has the sticky bit set, so if
it is world-writable as well it doesn't mean that the actual store path
is writable. Let alone the fact that it is only writable during the
build process.

This should fix installing the extension pack when enableExtensionPack
is used.

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