Make previous_weekly be the last successful DAG run of image_suggestions

https://airflow.apache.org/docs/apache-airflow/2.1.4/macros-ref.html?highlight=macros#default-variables

{{ prev_execution_date_success }} - execution date from prior successful dag run (if available) (pendulum.Pendulum)

We've been fighting with this for a bit, to now find that it was a Jinja template one liner... 🤦

Bug: T330688