types2: use strings.Builder in errors.go

types2: use strings.Builder in errors.go

Permissible now because go1.17 is used for bootstrapping.

Change-Id: Ia6f314f9314eda1cc42002dcbdfa551196df76f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/427554
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>