os/signal: skip nohup tests on darwin builders

os/signal: skip nohup tests on darwin builders

The new LUCI builders have a temporary limitation that breaks nohup.
Skip nohup tests there.

For #63875.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-arm64_13
Change-Id: Ia9ffecea7310f84a21f6138d8f8cdfc5e1392307
Reviewed-on: https://go-review.googlesource.com/c/go/+/538698
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>