support dual launch templates: spot for normal builds, on-demand for nested virtualization

AWS does not allow cpu_options.nested_virtualization with spot instances. Add a second
launch template (on-demand, cpu_options enabled) alongside the existing spot template.
The autoscaler selects the template per-system based on nested_virtualization config.

- RuntimeAdapter.launch_spot -> launch_instance(nested_virtualization=False)
- EC2Runtime: selects spot or on-demand LT; raises misconfiguration error if
  on_demand_launch_template_id is empty when nested_virtualization=True
- AwsConfig: add on_demand_launch_template_id field
- SystemConfig: add nested_virtualization field
- Scheduler: looks up system config to pass nested_virtualization flag
- NixOS module: new aws.onDemandLaunchTemplateIdFile + capacity.nestedVirtualization
  options; assertion prevents enabling nestedVirtualization without the LT ID file

agent/nix_builder_autoscaler/scheduler.py

@@ -245,8 +245,11 @@ def _launch_slot(
     """Launch a single slot. Transition to LAUNCHING on success, ERROR on failure."""
     slot_id = slot["slot_id"]
     user_data = render_userdata(slot_id)
+    system_name = slot.get("system", config.capacity.default_system)
+    sys_cfg = next((s for s in config.systems if s.name == system_name), None)
+    nested_virt = sys_cfg.nested_virtualization if sys_cfg else False
     try:
-        instance_id = runtime.launch_spot(slot_id, user_data)
+        instance_id = runtime.launch_instance(slot_id, user_data, nested_virtualization=nested_virt)
         metrics.counter("autoscaler_ec2_launch_total", {"result": "success"}, 1.0)
         db.update_slot_state(slot_id, SlotState.LAUNCHING, instance_id=instance_id)
         log.info("slot_launched", extra={"slot_id": slot_id, "instance_id": instance_id})