install_ffmpeg: Allow specifying ffmpeg installation root dir (#1948)
install_ffmpeg: Allow specifying ffmpeg installation root dir (#1948)

* install_ffmpeg: Allow changing installation path

* Replace the tilde with $ROOT as well

* install_ffmpeg: Force root to be /build on Windows

Stop changing the $HOME to something else though.
Let's see if that works.
livepeer/go-livepeer,livepeer/go-livepeer