Fixes the test on the linux-arm64-packet builder.

Change-Id: Icaf9edf9542f4f6e3791ca43298a1e7051eaa576
Reviewed-on: https://go-review.googlesource.com/c/go/+/182378
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>