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

gnanderson/go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go