proxy sumdb with first not-direct GOPROXY #1723
                
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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