The content-based staleness code means that

	go run -gcflags=-l helloworld.go

recompiles all of helloworld.go's dependencies with -gcflags=-l,
whereas before it would have assumed installed packages were
up-to-date. In this test, that means every race iteration rebuilds
the runtime and maybe a few other packages. Instead, install them
to a temporary location for reuse.

This speeds the test from 17s to 9s on my MacBook Pro.

Change-Id: Ied136ce72650261083bb19cc7dee38dac0ad05ca
Reviewed-on: https://go-review.googlesource.com/73992Reviewed-by: Ian Lance Taylor <iant@golang.org>