internal/socket: pass race detection if CGO is not enabled
internal/socket: pass race detection if CGO is not enabled

Follows up CL 205461 by recognizing that -race cannot
run if CGO is not enabled, hence won't broadly fail
the test.

Fixes golang/go#35668

Change-Id: I6a47e6fc7c4afd0bf225fc9876c96b29efa863ce
Reviewed-on: https://go-review.googlesource.com/c/net/+/207677
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/net