go/build: format with latest gofmt

Change-Id: I172af5c83a0d4d79ad627ba9dfe02dead23bf3a6
Reviewed-on: https://go-review.googlesource.com/130376
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>