vbox-guest: Merge pull request #9063 from ctheune.

This places mount.vboxsf in $out/bin instead of sbin, because as
reported in #9063 it seems that the the mount tools are no longer
looking into $storepath/sbin/mount.$what but into
$storepath/bin/mount.$what instead.

However, I haven't found any commit which changes this behavior and
couldn't reproduce it. Also, merging this will break the VirtualBox
tests, but I'm merging it anyway in an effort to remove $storepath/sbin
from virtualboxGuestAdditions entirely.

Tested against virtualbox NixOS VM test, which of course failed as said
before.