build: fix cross-compile on Plan 9

build: fix cross-compile on Plan 9

In Plan 9's shell,

	GOBIN= \
		foo bar

is the same as

	GOBIN=foo bar

Write what was meant, which is

	GOBIN=() \
		foo bar

Fixes #17737.

Change-Id: Ie5a1b51a7cec950b5e78bbbe99cbc3cfe102f980
Reviewed-on: https://go-review.googlesource.com/33144
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>