20-grub.install: use ~debug instead of -debug as suffix to sort correctly
    
    For the debug BLS entries a -debug suffix was added so they are sorted after
    the kernel entries, but that only works with version sort and not rpm sort.
    
    So instead use ~debug prefix so rpm sort algorithm could sort it correctly.
    
    Related: rhbz#1638103
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>