cmd/vet: use strings.Builder

cmd/vet: use strings.Builder

Change-Id: I583e167dac033037e82e55a9ae6c6f0c47a34c7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/428293
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tim King <taking@google.com>
Run-TryBot: Tim King <taking@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>