go/gcexportdata: fix test again (fix older builds)

This time, chose a package that's not changed across several builds.
Follow-up on https://go-review.googlesource.com/45151.

Change-Id: I0b487a45f8b12179b3aa495d852c0b7caa44c921
Reviewed-on: https://go-review.googlesource.com/45154
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed