log: don't format if writing to io.Discard

Fixes #47164

Change-Id: Ied03842360be4c86f1d9ead816f12c057a1f8dad
Reviewed-on: https://go-review.googlesource.com/c/go/+/348741


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Valentin Deleplace <deleplace@google.com>