build: don't use a fixed filename in temp
build: don't use a fixed filename in temp

Fixes issue 2688

R=golang-dev, gri, rsc
CC=golang-dev
http://codereview.appspot.com/5539050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go