cmd/go: fix tests when go.env sets GOPROXY=direct GOSUMDB=off

cmd/go: fix tests when go.env sets GOPROXY=direct GOSUMDB=off

Tested locally by changing GOROOT/go.env. At some point perhaps we
should also set up a builder that runs with some common expected
modifications to go.env
(such as GOTOOLCHAIN=local GOPROXY=direct GOSUMDB=off).

Fixes #61358.
Updates #61359.

Change-Id: I365ec536bec86370e302fb726fa897400ab42cf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/509637
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>