cmd/internal/moddeps: set GO111MODULE explicitly for moddeps_test 'go' commands

This fixes observed failures using the following steps to reproduce:

	go env -w GO111MODULE=off
	go test cmd/internal/moddeps

Fixes #37749

Change-Id: I7761f0b20266ac911ad19a724ba2551beca3f267
Reviewed-on: https://go-review.googlesource.com/c/go/+/222674


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>