runtime/debug: update SetCrashOutput example to not pass parent env vars

Fixes #73490

Change-Id: I500fa73f4215c7f490779f53c1c2c0d775f51a95
Reviewed-on: https://go-review.googlesource.com/c/go/+/667775


Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>