cmd/gorename: set GO111MODULE=off in gorename_test

Updates golang/go#27858

Change-Id: Ia823c49d8f05831c833df9f745a879c94c915626
Reviewed-on: https://go-review.googlesource.com/c/139319
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

cmd/gorename/gorename_test.go

@@ -48,6 +48,8 @@ func TestGeneratedFiles(t *testing.T) {
 			env = append(env, envVar)
 		}
 	}
+	// gorename currently requires GOPATH mode.
+	env = append(env, "GO111MODULE=off")
 
 	// Testing renaming in packages that include cgo files:
 	for iter, renameTest := range []test{