gopack: remove warning from the use of mktemp
gopack: remove warning from the use of mktemp

gcc-4.6 considers mktemp to be racey

R=golang-dev, rsc, iant
CC=golang-dev
http://codereview.appspot.com/4602050

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang