net: skip DNS tests on self-hosted Android

They were already skipped on tethered Android builders because
the tests are gated on GO_BUILDER_NAME being set and the Android
exec wrapper does not propagate GO_BUILDER_NAME.

Updates #31722

Change-Id: Ifd2c7daecc19a4e540d86d1f38083f43cc3e6b15
Reviewed-on: https://go-review.googlesource.com/c/go/+/175177


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>