FFmpeg 4 fails to install in CI

[scotts]

FFmpeg 4 has failed to install in our CI for a while now; we know just to ignore these failures. We should:

1. Investigate what's going on, briefly. If we can fix the issue after a brief investigation, great.
2. If the brief investigation isn't conclusive, we should disable FFmpeg 4 tests for now.
3. Prioritize the longer-term fix.

Note that this may be related to #325, where FFmpeg 5 tests were failing in a similar way. We ended up just disabling the tests.

Example failure: https://github.com/pytorch/torchcodec/actions/runs/15781205932/job/44488022132?pr=725