handle_welcome_screen: only set _WELCOME_DONE if we saw it
    
    This is to handle a temporary condition where the screen isn't
    present on the KDE base disk images for F38 or F39 yet, so they
    only see it on the second boot on update tests, but don't handle
    it because we marked it as already 'done'.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>