can_list_to_can_capnp_cpp: write message directly to the output string (#19912)

selfdrive/boardd/can_list_to_can_capnp.cc

@@ -22,8 +22,10 @@ void can_list_to_can_capnp_cpp(const std::vector<can_frame> &can_list, std::stri
     c.setDat(kj::arrayPtr((uint8_t*)it->dat.data(), it->dat.size()));
     c.setSrc(it->src);
   }
-  auto bytes = msg.toBytes();
+  const uint64_t msg_size = capnp::computeSerializedSizeInWords(msg) * sizeof(capnp::word);
-  out.append((const char *)bytes.begin(), bytes.size());
+  out.resize(msg_size);
+  kj::ArrayOutputStream output_stream(kj::ArrayPtr<capnp::byte>((unsigned char *)out.data(), msg_size));
+  capnp::writeMessage(output_stream, msg);
 }
 
 }