When finding the pkg dir, androidtest.bash assumes
that GOARCH is set. Require it up front.

Change-Id: I143f7b59ad9d98b9c3cfb53c1d65c2d33a6acc12
Reviewed-on: https://go-review.googlesource.com/27926
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>