createiso: Use unique paths for breaking hardlinks
    
    If the data needs to be split into multiple ISOs, we need to make sure
    the paths are unique for each image. Otherwise all files will be copied
    into the same directory, and once the first image is finished, the whole
    staging dir is deleting. That obviously breaks the tasks that are still
    in progress.
    
    JIRA: COMPOSE-2610
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>