I also agree with reducing (increasing?) single points of failure. I'm not trying to be pedantic, but rather observing that in practice, it's not nearly as easy as spinning up a backup Git server (which is already hard enough).
Maintaining two classes of build infrastructure throughout all your dependencies is probably not a worthwhile problem to solve, unless you want to control for the improbable risk that GitHub will be down for weeks at a time. You'd be much better off ensuring that you are able to perform rollbacks without needing to pull from the external world, because this way the worst case scenario is you run a stale version for the time that GitHub is down, in the off chance you pushed a broken version right before the outage.