Allow pining ffmpeg version in download_ffmpeg

[hongbo-miao]

Thanks for the convenient library.
It would be great to allow pining ffmpeg version in download_ffmpeg, this would help production more predictable.
Thanks! 😃

[nathanbabcock]

Hey @hongbo-miao, good suggestion, thank you.

I took a look at our build providers and most of them provide old binaries going back to FFmpeg 5.x or earlier. There might be some edge cases but that's okay.

I'll look into adding this in the next release. If anyone needs it sooner or is willing to contribute please feel free to start a PR