Reportedly waitid is not available for Ubuntu on Windows.

Fixes #16610.

Change-Id: Ia724f45a85c6d3467b847da06d8c65d280781dcd
Reviewed-on: https://go-review.googlesource.com/25507
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>