When raw_exec is configured with [`no_cgroups`](https://www.nomadproject.io/docs/drivers/raw_exec#no_cgroups), raw_exec shouldn't attempt to create a cgroup.

Prior to this change, we accidentally always required freezer cgroup to do stats PID tracking. We already have the proper fallback in place for metrics, so only need to ensure that we don't create a cgroup for the task.

Fixes https://github.com/hashicorp/nomad/issues/8565