cmd/go: stop using $GOROOT and $GOBIN in script output
cmd/go: stop using $GOROOT and $GOBIN in script output

They were necessary to produce a canonical script
when we checked in build scripts, but now they're just
getting in the way.

Fixes issue 3279.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5796068
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go