net/http/cgi: preserve LD_LIBRARY_PATH on NetBSD, too

net/http/cgi: preserve LD_LIBRARY_PATH on NetBSD, too

This makes the behavior of the cgi package on NetBSD consistent with its
behavior on the other BSDs. It is also necessary for the test suite to
pass on NetBSD with gccgo (see CL 261137).

Change-Id: I531a6e0954f895a921204dcc61e3f3e90860a23d
Reviewed-on: https://go-review.googlesource.com/c/go/+/263577
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>