hydra-queue-runner: Support running in a NixOS container

In a NixOS container, cmdBuildDerivation doesn't work because we're
not privileged. But we also don't need it because the store already
has the derivation.

Also, don't copy from/to the store since this gives errors about
missing signatures.

src/hydra-queue-runner/state.hh

@@ -274,6 +274,11 @@ struct Machine
 
         return true;
     }
+
+    bool isLocalhost()
+    {
+        return sshName == "localhost";
+    }
 };