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/logging.py

@@ -12,7 +12,17 @@ from typing import Any
 class JSONFormatter(logging.Formatter):
     """Format log records as single-line JSON."""
 
-    EXTRA_FIELDS = ("slot_id", "reservation_id", "instance_id", "request_id", "error", "category", "count", "ids", "idle_seconds")
+    EXTRA_FIELDS = (
+        "slot_id",
+        "reservation_id",
+        "instance_id",
+        "request_id",
+        "error",
+        "category",
+        "count",
+        "ids",
+        "idle_seconds",
+    )
 
     def format(self, record: logging.LogRecord) -> str:
         """Format a log record as JSON."""