Doesn't get defaulted if $HOME/go is a GOROOT.

Change-Id: I6ac8211a74029e4ad70a50f6e7884a039a27ab05
Reviewed-on: https://go-review.googlesource.com/33720
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>