log retention env var name misleading

[jaketf]

ap-airflow/2.0.2/buster/include/clean-airflow-logs

Line 6 in ab60218

RETENTION=${ASTRONOMER__AIRFLOW__WORKER_LOG_RETENTION_DAYS:-15}

This environment variable implies that the retention is only relevant to the worker logs. however this script is also used to clean the scheduler logs in the helm chart. These should either be controlled by separate variables or this name should change to be ASTRONOMER__AIRFLOW__LOG_RETENTION_DAYS