workstation: Don't autostart gnome-software on live media
    
    gnome-software session service automatically downloads yum metadata,
    which all goes to the RAM-backed filesystem overlay when running the
    live image. This is undesired as it can make it difficult to install
    Workstation on low memory devices.
    
    To fix this, this commit disables gnome-software xdg autostart service
    and the gnome-shell search provider on the live media.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1560504