restore ffmpeg 4.4

- blocked by conda-forge/ffmpeg-feedstock#100

In #76, we dropped `ffmpeg` from the `tools` package as the `conda-forge` version wasn't being correctly skipped, and is insufficient for the test output. Should restore, once available.

```
__________________________________ test_diff ___________________________________

differences = []

@skip_unless_tools_exist("ffprobe")
@skip_unless_tool_is_at_least("ffprobe", ffprobe_version, "4.4")
def test_diff(differences):
> assert_diff(differences[0], "mp3_expected_diff")

tests/comparators/test_ffprobe.py:57:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

difference = , filename = 'mp3_expected_diff'

def assert_diff(difference, filename):
> assert difference.unified_diff == get_data(filename)
E AssertionError

tests/utils/data.py:50: AssertionError
```