Feature Request: Override ffmpeg path using environmental variable

[DeftNerd]

I'm stuck on Ubuntu 18.04 which has an older version of ffmpeg. I've been able to install a snap package with the latest ffmpeg version, but it lives somewhere else.

Using 'which' to find ffmpeg is great, but it would be helpful to be able to set an environmental variable with a path to an alternate ffmpeg location.

This could also be useful for people who compile versions from source to test.

[kirill-konshin]

Especially useful if you install ffmpeg via NPM: npm install ffmpeg-cli for example.