Fix path to nologin shell for libvirt-qemu user on Debian/Ubuntu
    
    Unfortunately, Debian/Ubuntu systems are not guaranteed to have
    a consistent file path to the nologin shell because while
    UsrMerge/UsrMove is applied by default since Debian 10 and
    Ubuntu 19.04, systems manually upgraded from earlier versions
    did not necessarily pick up this change without direct intervention,
    which has not happened yet.
    
    For now, this means that the nologin shell does not actually exist
    in /sbin, but instead in /usr/sbin.