net: skip TestUnixConnLocalWindows on windows/386

Recent CL 125456 implemented Unix Socket functionality on windows.
But that functionality does not appear to be working when 32-bit
code is used. So disable TestUnixConnLocalWindows.

windows/386 builder does not appear to be complaining about
TestUnixConnLocalWindows, because new functionality requires
Windows 10 Build 17063. windows/386 builder uses Windows 2008.

Fixes #27943

Change-Id: Iea91b86aaa124352d198ca0cd03fff1e7542f949
Reviewed-on: https://go-review.googlesource.com/138676


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>