cmd/go: clarify GODEBUG in go help environment

Fixes #37004
Fixes #50444

Change-Id: I7dd5a8c9bd0a2122ff38508cf509369d6d8ad599
Reviewed-on: https://go-review.googlesource.com/c/go/+/632177


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>