cmd/go: document that GOENV=off disables the default config file
Fixes #46840
Change-Id: Icc5475e312003e316dc039413a35089485163ba6
Reviewed-on: https://go-review.googlesource.com/c/go/+/381894
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Trust: Daniel Martà <mvdan@mvdan.cc> Reviewed-by: Daniel Martà <mvdan@mvdan.cc> Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>