Otherwise the packages will not pick up the way that -installsuffix is
changed by -buildmode and -race.

Fixes #20512.

Change-Id: Ieca32c958a531beb6331dff30d7e9a4998867dbb
Reviewed-on: https://go-review.googlesource.com/44630
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>