cmd/go/internal/modfetch: fix GOSUMDB test failures
Use cfg.GOSUMDB consistently instead of re-resolving it from the environment.
Set cfg.GOSUMDB to 'off' explicitly in coderepo_test, since it may
include modules that cannot be fetched using a released version of the
'go' command.
Fixes #31964
Change-Id: I17cae9e0c6aa1168ba534e6da4e3652800ac81e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/176538
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>