CLI: fix download-task's session typo
    
    Koji CLI calls on like this:
    
        koji -d download-task $TASK_ID
    
    which involve getTaskChildren() XMLRPC call fail:
    
        ...
        File "/usr/bin/koji", line 6460, in anon_handle_download_task
            base_task = session.getTaskInfo(base_task_id)
        NameError: global name 'session' is not defined