build: move $GOBIN ahead of /bin, /usr/bin in build $PATH
build: move $GOBIN ahead of /bin, /usr/bin in build $PATH

Otherwise if there is an installed /usr/bin/6g the build
will use that one instead of the one it builds.

R=r
CC=golang-dev
http://codereview.appspot.com/4249045
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go