cmd/go: set GO111MODULE=off explicitly in TestScript/list_test_err

This test was added after CL 162697.

Updates #30228

Change-Id: Ia33ad3adc99e53b0b03e68906dc1f2e39234e2cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/166697
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/testdata/script/list_test_err.txt

+env GO111MODULE=off
+
 # issue 28491: errors in test source files should not prevent
 # "go list -test" from returning useful information.