Import os in executor_loader (apache#44927)

That import was removed in apache#44839, but apache#44710 wasn't up-to-date with main so
static checks there didn't fail. This simply adds it back.

airflow/executors/executor_loader.py

@@ -19,6 +19,7 @@
 from __future__ import annotations
 
 import logging
+import os
 from typing import TYPE_CHECKING
 
 from airflow.exceptions import AirflowConfigException, UnknownExecutorException