sim: fix gps message (#25521)

fix gps timestamp field renamed

bug introduced in https://github.com/commaai/cereal/pull/341
old-commit-hash: f65547fbe2

tools/sim/bridge.py

@@ -179,7 +179,7 @@ def gps_callback(gps, vehicle_state):
   ]
 
   dat.gpsLocationExternal = {
-    "timestamp": int(time.time() * 1000),
+    "unixTimestampMillis": int(time.time() * 1000),
     "flags": 1,  # valid fix
     "accuracy": 1.0,
     "verticalAccuracy": 1.0,