cmd/go: install binaries in cmd/ in $GOROOT/bin instead of $GOBIN

This was caused by CL 167748, which removed a special case for command
line arguments starting with "cmd/". This CL restores the behavior
from go1.12.

Fixes #32674

Change-Id: I72180d11fb0261ef0af9632e512bd9c03481b6c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/183058


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>