task lifecycle: long running poststop task with dynamic template data can hang

### Nomad version

1.3.3

### Operating system and Environment details

Ubuntu 20.04
3 Servers
3 Clients
Single Region

### Issue

A long running `poststop` task that references dynamic data can cause an allocation to never stop if the dynamic data keeps changing before the long-running `poststop` task can complete.

### Reproduction steps

First, create some sort of dynamic data that can be referenced by a job template. This example will use a Vault PKI secret. Notice the `max_ttl=15s` arg passed for `my-path/roles/nomad`.

```hcl
vault secrets enable -path=my-path pki

vault write my-path/root/generate/internal common_name=service.consul ttl=1h

# Notice the max_ttl below
vault write my-path/roles/nomad \
allowed_domains=service.consul \
allow_subdomains=true \
generate_lease=true \
max_ttl=15s

vault secrets tune -max-lease-ttl=1m my-path

# create secrets policy
vault policy write policy-my-path -<> "${NOMAD_ALLOC_DIR}/lifecycle"
while true; do
sleep 1
done
EOF
destination = "local/script.sh"
}

resources {
cpu = 50
memory = 50
}
}

task "poststop" {
driver = "raw_exec"

lifecycle {
hook = "poststop"
}

config {
command = "/bin/bash"
args = ["local/script.sh"]
}

template {
data = <> "${NOMAD_ALLOC_DIR}/lifecycle"
sleep 100
EOF
destination = "local/script.sh"
}

resources {
cpu = 50
memory = 50
}
}
}
}
```

Now run `job stop -purge` on the job, and watch the `poststop` task from the UI.

#### Expected Result

This works as designed, but it illustrates the need for a way to configure the task so that the allocation does not get hung trying to stop.

#### Actual Result

The task will continually restart every `15s` due to the `max_ttl` on the Vault secret causing the template to re-render which triggers a task restart. The `poststop` task will never finish, and the allocation will continue to run.

Start by reproducing the issue with the provided Vault PKI setup, jobspec, and `job stop -purge`, then trace the poststop task lifecycle when dynamic template data triggers restarts. Done means a configurable behavior prevents the poststop task from continually restarting and allows the allocation to stop while preserving the intended template handling.

Written by the indexing model from the issue text.