You can also pick your own proxy primary and fallbacks.

For example, this should in theory work to make the google-run proxy your fourth choice, with three other proxies attempted before the google-run proxy, and with direct (no proxy) access as the fifth and final fallback:

  go env -w GOPROXY=gocenter.io,goproxy.io,goproxy.cn,proxy.golang.org,direct

There is some geographic diversity in that example as well -- I think two mirrors in that list are primarily run in China.