Another attempt at https://golang.org/cl/5754088.

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.

Tested a few cases in test.bash.

Ran all.bash with and without $GOBIN and it works.
Even so, I expect it to break the builders,
like it did last time, we can debug from there.

Fixes #3269 (again).
Fixes #3396.
Fixes #3397.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5927051