fix: add missing task-langs attribute

[guenthermi]

Currently the command line tool is not working because of a change made by #131. This might be fixed if the refactoring of the command line tool propose in #7 will be ever merged. However, since this PR is work in progress for over one year and not finished. I made this small fix.

At the moment you get the following error message when you try to run the mteb comment (version on main branch) because args.task_langs does not exsists:

Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/michael/workspace/mteb-long-documents/mteb/cmd.py", line 124, in <module>
    main()
  File "/home/michael/workspace/mteb-long-documents/mteb/cmd.py", line 117, in main
    task_categories=args.task_categories, task_types=args.task_types, task_langs=args.task_langs, tasks=args.tasks
AttributeError: 'Namespace' object has no attribute 'task_langs'