cmd/go: fix testdata/script/mod_vendor_goversion for new compiler output

The message is now a single line, but I couldn't get '\n?' to work.

(This test does not run in short mode.)

Change-Id: I3d7033f78ac2bf41db7fdfe96cbb988c4533d834
Reviewed-on: https://go-review.googlesource.com/c/go/+/359095


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>