text/template: use strings.Builder

text/template: use strings.Builder

Change-Id: I559986b5408d3967c58c9abc6e8f4b04b25496a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/428280
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@google.com>