boardd: declare MessageBuilder close to usage (#19883)

old-commit-hash: 2df9736bb2

selfdrive/boardd/boardd.cc

@@ -287,9 +287,6 @@ void can_health_thread() {
 
   // run at 2hz
   while (!do_exit && panda->connected) {
-    MessageBuilder msg;
-    auto healthData = msg.initEvent().initHealth();
-
     health_t health = panda->get_health();
 
     if (spoofing_started) {
@@ -349,6 +346,8 @@ void can_health_thread() {
     uint16_t fan_speed_rpm = panda->get_fan_speed();
 
     // set fields
+    MessageBuilder msg;
+    auto healthData = msg.initEvent().initHealth();
     healthData.setUptime(health.uptime);
 
 #ifdef QCOM2