ffmpeg is installed, but stempeg/cmds.py is saying it isn't

Howdy! I'm having trouble using `stempeg` on a fresh Ubuntu machine. I installed and built `ffmpeg` using script that was linked to in the README, and `ffmpeg` sits in `/usr/bin` and works...
```bash
$ which ffmpeg
/usr/bin/ffmpeg
$ ffmpeg --help
ffmpeg version 4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 11 (Ubuntu 11.4.0-1ubuntu1~22.04)
configuration: --prefix=/root/ffmpeg_build --extra-cflags=-I/root/ffmpeg_build/include --extra-ldflags=-L/root/ffmpeg_build/lib --bindir=/root/bin --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-nonfree
libavutil 56. 31.100 / 56. 31.100
libavcodec 58. 54.100 / 58. 54.100
libavformat 58. 29.100 / 58. 29.100
libavdevice 58. 8.100 / 58. 8.100
libavfilter 7. 57.100 / 7. 57.100
...etc
```

Same as on my old machine where everything's fine.

But when I run the python code on my new machine, I get...

```python
File "/mnt/sfs/shawley.../data.py", line 2, in
import stempeg
File "/home/shawley/envs/myenv/lib/python3.10/site-packages/stempeg/__init__.py", line 16, in
from .read import read_stems
File "/home/shawley/envs/myenv/lib/python3.10/site-packages/stempeg/read.py", line 7, in
from stempeg.write import FilesWriter
File "/home/shawley/envs/myenv/lib/python3.10/site-packages/stempeg/write.py", line 23, in
from .cmds import FFMPEG_PATH, mp4box_exists, get_aac_codec, find_cmd
File "/home/shawley/envs/myenv/lib/python3.10/site-packages/stempeg/cmds.py", line 43, in
raise RuntimeError(
RuntimeError: ffmpeg or ffprobe could not be found! Please install them before using stempeg. See: https://github.com/faroit/stempeg
```
Setting `export FFMPEG_PATH=/usr/bin` or even `export FFMPEG_PATH=/usr/bin/ffmpeg` doesn't help.

What needs to happen here?
Thanks!