net: demote TestDialerDualStack to a flaky test

Only run TestDialerDualStack on the builders, as to not annoy or
otherwise distract users when it's not their fault.

Even though the intention is to only run this on the builders, very
few of the builders have IPv6 support. Oh well. We'll get some
coverage.

Updates #13324

Change-Id: I13e7e3bca77ac990d290cabec88984cc3d24fb67
Reviewed-on: https://go-review.googlesource.com/24985


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>