proxy sumdb with first not-direct GOPROXY

[jan4984]

What is the problem I am trying to address?

athens is not working for people who can not access https://sum.golang.org. A go mod proxy should also proxy sumdb requests.

How is the fix applied?

redirect sum.golang.org sumdb to first not-direct proxy set in GOPROXY, like https://goproxy.cn

What GitHub issue(s) does this PR fix or close?

Fixes #1468