osbs: Fresh koji session for getting metadata
    
    The task can take a while to finish, and it's possible for the
    authenticated session to expire in the mean time. Watching the task will
    work, because that happens by spawning `koji watch-task` as subprocess.
    
    We can work around this by creating a fresh unauthenticated session for
    getting the task results.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>