What's the local workaround for this?

Git is distributed, it should be possible to put something between our servers and github which pulls from github when it's running and otherwise serves whatever it used to have. A cache of some sort. I've found the five year old https://github.com/jonasmalacofilho/git-cache-http-server which is the same sort of idea.

I've run a git instance on a local machine which I pull from, where a cron job fetches from upstream into it, which solved the problem of cloning llvm over a slow connection, so it's doable on a per-repo basis.

I'd like to replace it globally though because CI looks like "pull from loads of different git repos" and setting it up once per-repo seems dreadful. Once per github/gitlab would be a big step forward.