Retry flaky ffmpeg download step in CI

[Copilot]

The setup-ffmpeg step occasionally fails due to transient network issues, causing CI to fail for reasons unrelated to code changes.

Changes

• Wrap FedericoCarboni/setup-ffmpeg@v3 with Wandalen/wretry.action@v3.8.0 to automatically retry up to 3 times with a 5s delay between attempts

- name: Set up FFmpeg
  uses: Wandalen/wretry.action@v3.8.0
  with:
    action: FedericoCarboni/setup-ffmpeg@v3
    attempt_limit: 3
    attempt_delay: 5000
    with: '{ "ffmpeg-version": "release" }'