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

hive-c0re/src/attic/closure.rs

@@ -174,6 +174,7 @@ mod tests {
             built_unix: 1,
             cached,
             private_users: false,
+            nix_builds: hive_types::NixBuilds::Host,
         }
     }
 

hive-c0re/src/attic/probe.rs

@@ -246,6 +246,7 @@ mod tests {
             built_unix: 1,
             cached: true,
             private_users: false,
+            nix_builds: hive_types::NixBuilds::Host,
         }
     }