Worker will now try one of the following paths, relative to the flamenco-worker
executable, in order to find FFmpeg. If they cannot be found, `$PATH` is
searched for FFmpeg.
- `tools/ffmpeg-$GOOS-$GOARCH`
- `tools/ffmpeg-$GOOS`
- `tools/ffmpeg`
On Windows these paths will have a `.exe` suffix appended. `$GOOS` is the
operating system, like "linux", "darwin", "windows", etc. `$GOARCH` is the
architecture, like "amd64", "386", etc.