I fear the github plugin will not help you out here much as it requires the library to be the repo not some subpath of the repo (which is why we ditched it for the official lib)
My suggestion is to clone the git repo in the usual manner to a local path and integrate that local path into your setup like any other local library. (This has the added benefit that you are able to control when you update the library or if you even want to use the master branch at all.)
The appendix of Library management in KiCad version 5 contains a description on how to do it for the official library which should be a good guide should you need help with this.