cmd/go: improve pseudo-version timestamp error

The previous "invalid pseudo-version: does not match version-control
timestamp" error message used a different timestamp format than the
format used in go.mod and go.sum. For cut-and-paste-ability this patch
makes the two consistent.

Fixes #36974

Change-Id: I21f344ab9898cc584c0bcf4a75d74275a703c650
Reviewed-on: https://go-review.googlesource.com/c/go/+/217437


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>