fix: escape $ in airflow-init to properly set AIRFLOW_UID (apache#52988)

Author: givemechocopy

airflow-core/docs/howto/docker-compose/docker-compose.yaml

@@ -217,7 +217,7 @@ services:
           echo "For other operating systems you can get rid of the warning with manually created .env file:"
           echo "    See: https://airflow.apache.org/docs/apache-airflow/stable/howto/docker-compose/index.html#setting-the-right-airflow-user"
           echo
-          export AIRFLOW_UID=$(id -u)
+          export AIRFLOW_UID=$$(id -u)
         fi
         one_meg=1048576
         mem_available=$$(($$(getconf _PHYS_PAGES) * $$(getconf PAGE_SIZE) / one_meg))