Add `ffmpeg` for `doc_test_job` on CircleCI
# What does this PR do?

Need this at least for `docs/source/en/task_summary.md`.

Otherwise, [job](https://app.circleci.com/pipelines/github/huggingface/transformers/66845/workflows/ae6bcd25-5071-4f48-a9ba-d446ae6e060f/jobs/833148) fails
_The documentation is not available anymore as the PR was closed or merged._