Update the apply_cirun workflow to only trigger on a synchronize event. (#1440)
Update the apply_cirun workflow to only trigger on a synchronize event. (#1440)

With https://github.com/tensorflow/tflite-micro/pull/1424, we are triggering all the workflows directly from the `ci:ready_to_merge_label`.

As a result, we no longer need to apply ci:run from a pull_request:labeled event.

Additionally, with the apply_cirun workflow in its current form, when a reviewer adds the `ci:ready_to_merge` label the workflow ends up removing it which interferes with the merge queue.

The correct behavior is to only automatically apply ci:run on a synchronize event (when the event comes from mergify).

BUG=http://b/246664234
tensorflow/tflite-micro,tensorflow/tflite-micro,tensorflow/tflite-micro,tensorflow/tflite-micro