cmd/dist: add GOBIN to env's output
cmd/dist: add GOBIN to env's output

clean.bash depends on it being set.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5630056
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go