The standard build assumes the variable is unset.
Make it so, like we do for GOFLAGS, GOBIN, and so on.

Change-Id: I4ad5695f8021b08bd1a35dd99112970a813d247c
Reviewed-on: https://go-review.googlesource.com/c/149959
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>