cmd/go: update error expectations in TestScript/mod_install_pkg_version

cmd/go: update error expectations in TestScript/mod_install_pkg_version

This test was missed in CL 298650, and not caught by TryBots because
it is skipped it short mode (and we forgot to add longtest TryBots on
that CL).

Updates #44745

Change-Id: I696d01307dabf351b0e4735db0644f4e09c8e369
Reviewed-on: https://go-review.googlesource.com/c/go/+/298794
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>