Do run it on the builders, though.

Fixes #15881.

Change-Id: Ie42204d553cb18547ffd6441afc261717bbd9205
Reviewed-on: https://go-review.googlesource.com/24111
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>