net: make mustHaveExternalNetwork work as usual on GOOS=linux

I considered deleting mustHaveExternalNetwork in favor of just using
the real testenv.MustHaveExternalNetwork. That certainly makes these
tests that call it easier to understand. But that negatively affects
some ports that don't have a longtest builder as it'd make the tests
not run automatically on any builder at all.

So, make a minimal change that applies only to GOOS=linux for now.
If we make more progress on establishing -longtest builders for all
ports, this intermediate layer helper will cease to have any benefit
and can be deleted in favor of the one in testenv package.

Change-Id: Iaea207d98e780db429ab49e6e227650a8b35b786
Reviewed-on: https://go-review.googlesource.com/c/go/+/513416
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

0 files changed, 0 insertions, 0 deletions