agent: log task chain properly

Rather than this:

```
Sep 03 23:44:43 core-01 fleetd[1467]: INFO reconcile.go:267: AgentReconciler task chain failed: chain={%!s(*job.Unit=&{foo.service {map[] []} }) [{UnloadUnit purging agent}]} err=task already in flight
```

This PR logs this:

```
Sep 03 23:44:43 core-01 fleetd[1467]: INFO reconcile.go:267: AgentReconciler task chain failed: chain={foo.service (UnloadUnit, "purging agent")} err=task already in flight
```