[release-branch.go1.15] cmd/go: fix get_update_unknown_protocol test

This test needs to run in GOPATH mode. It broke when a go.mod file was
added to github.com/golang/example. This change sets GO111MODULE=off,
which matches master since CL 255051.

Fixes #43861

Change-Id: I9ea109a99509fac3185756a0f0d852a84c677bf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/285956


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>