fix replay unknown substitution arg

ouster-ros/launch/replay.composite.launch.xml

@@ -3,8 +3,8 @@
   <set_parameter name="use_sim_time" value="true" />
 
   <arg name="loop" default="false" description="request loop playback"/>
-  <arg if="$(arg loop)" name="_loop" value="--loop"/>
-  <arg unless="$(arg loop)" name="_loop" value=" "/>
+  <arg if="$(var loop)" name="_loop" value="--loop"/>
+  <arg unless="$(var loop)" name="_loop" value=" "/>
 
   <remap from="/os_node/imu_packets" to="/ouster/imu_packets"/>
   <remap from="/os_node/lidar_packets" to="/ouster/lidar_packets"/>
@@ -106,7 +106,7 @@
 
   <executable if="$(var _use_bag_file_name)" output="screen"
     launch-prefix="bash -c 'sleep 3; $0 $@'"
-    cmd="ros2 bag play $(var bag_file) --clock $(arg _loop)
+    cmd="ros2 bag play $(var bag_file) --clock $(var _loop)
       --qos-profile-overrides-path
       $(find-pkg-share ouster_ros)/config/metadata-qos-override.yaml"/>