[build-test-ostree] update pungi-make-ostree command
    
    pungi-make-ostree script is changed to support sub commands, for
    composing OSTree repo, should use 'tree' subcommand, and repo path is
    specified by '--repo' instead of positional argument now.
    
    The change should go out with pungi-4.1.12.
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>