[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>