syscall: skip TestDeathSignalSetuid if exec fails with a permission error

syscall: skip TestDeathSignalSetuid if exec fails with a permission error

Also explicitly look up user "nobody" (or "gopher" on the Go builders)
if running as root, instead of hard-coding UID/GID 99.

Fixes #62719.

Change-Id: I9fa8955f2c239804fa775f2478a5274af9330822
Reviewed-on: https://go-review.googlesource.com/c/go/+/529795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>