Systemd template file doesn't spawn unique agent names

The [template unit file](https://github.com/buildkite/agent/blob/main/packaging/linux/root/usr/share/buildkite-agent/systemd/buildkite-agent%40.service) intended for spawning multiple agents on one host doesn't start agents using unique names.

As the [documentation](https://buildkite.com/docs/agent/v3/ubuntu#running-multiple-agents) states, you can run

```bash
sudo systemctl enable --now buildkite-agent@1
sudo systemctl enable --now buildkite-agent@2
```

But the [default configuration file](https://github.com/buildkite/agent/blob/main/packaging/linux/root/usr/share/buildkite-agent/buildkite-agent.cfg#L5) does not use the `%n` substitution.

I'm not sure if the the templated file is intended to be used with a number that tells it how many agents to spawn or a string that will be used in the agents name. ie

```bash
# spawn four agents with incrementing names using %n
sudo systemctl enable --now buildkite-agent@4

# or, run multiple units spawning one agent each, using the template to inject the unique name
sudo systemctl enable --now buildkite-agent@deploy
sudo systemctl enable --now buildkite-agent@build
```

Inspect packaging/linux/root/usr/share/buildkite-agent/systemd/buildkite-agent@.service and compare it with packaging/linux/root/usr/share/buildkite-agent/buildkite-agent.cfg#L5. Use the documented systemctl commands and the Ubuntu multiple-agent documentation to determine whether the instance identifier represents a count or an agent name. Done means templated agents start with distinct, intended names in both documented usage patterns.

Written by the indexing model from the issue text.