Fixes TestFindStdlib in x/tools on android.

Change-Id: I2da7c702164e23488c7f9574f636ac36f63ab421
Reviewed-on: https://go-review.googlesource.com/c/go/+/167799
Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>