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 <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@google.com>
Reviewed-by: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@google.com>
Run-TryBot: hopehook <63023fcaeafd0730eb709923fc3c0482e6977249@golangcn.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go