add a per-agent nix builds mode, host or none, carried like privateUsers

swarm-builder/Cargo.toml

@@ -14,6 +14,8 @@ async-nats.workspace = true
 # StreamExt::next on the subscription.
 futures-util.workspace = true
 hive-log.workspace = true
+# `NixBuilds`, the type of the reply's `nix_builds`.
+hive-types.workspace = true
 # The build metrics, exported over OTLP like the review classifier's.
 opentelemetry.workspace = true
 opentelemetry-otlp.workspace = true