Add "cancel_old_ci" to Yamato config

By default, Yamato will run on every commit added to a PR after the PR is created. We'd rather just have it run on the latest commit.

Author: mrpropellers

.yamato/yamato-config.yml

@@ -4,11 +4,11 @@ agent:
     image: robotics/ci-ubuntu20:latest
     flavor: i1.medium
 commands:
-    - echo "Hello world!"
     - source /opt/ros/noetic/setup.bash && echo "ROS_DISTRO == $ROS_DISTRO" &&
         cd .. && mkdir -p catkin_ws/src && cp -r ./ROS-TCP-Endpoint catkin_ws/src &&
         cd catkin_ws && catkin_make && source devel/setup.bash && python3 -m pytest
 triggers:
+    cancel_old_ci: true
     expression: |
         pull_request.target in ["dev", "main"] AND
         NOT pull_request.push.changes.all match "**/*.md" AND