broke builders

««« original CL description
cmd/go: respect $GOBIN always

Before, we only consulted $GOBIN for source code
found in $GOROOT, but that's confusing to explain
and less useful.  The new behavior lets users set
GOBIN=$HOME/bin and have all go-compiled binaries
installed there.

Fixes #3269.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5754088
»»»

TBR=bradfitz
CC=golang-dev
https://golang.org/cl/5794065