cmd/stringer: add temporary(?) fix to get stringer test working in module mode

Updates golang/go#30874

Change-Id: I170ea99b3f06c8bc4cc624e86b80eeb852ea7115
Reviewed-on: https://go-review.googlesource.com/c/tools/+/167771
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/stringer/endtoend_test.go

@@ -175,5 +175,6 @@ func runInDir(dir, name string, arg ...string) error {
 	cmd.Dir = dir
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
+	cmd.Env = append(os.Environ(), "GO111MODULE=auto")
 	return cmd.Run()
 }