cmd/go: document GO{NO}SUMDB in 'go help environment' and 'go help modules'

GOSUMDB and GONOSUMDB are described in detail by
'go help module-auth'. So, this change mentions the two
variables and says to see 'go help module-auth'.

This also adds GONOPROXY to 'go help environment'.

Fixes #32292 and updates #32056.

Change-Id: I2eae0f906a3fbfcb88ad5c8fb4870917d0d7bbcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/179219


Reviewed-by: Russ Cox <rsc@golang.org>