cmd/go: adjust expected error message in TestScript/mod_invalid_version

CL 188817 improved the error message for a nonexistent version in a dependency.
This CL locks in that improvement in a regression test.

Fixes #33474

Change-Id: I6246b4995adee966f24eaebe491d35830aea8370
Reviewed-on: https://go-review.googlesource.com/c/go/+/188977


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>