[AIRFLOW-6801] Make use of ImportError.timestamp (#7425)

(cherry picked from commit cc1bd64)

airflow/jobs/scheduler_job.py

@@ -608,6 +608,7 @@ def update_import_errors(session, dagbag):
         for filename, stacktrace in six.iteritems(dagbag.import_errors):
             session.add(errors.ImportError(
                 filename=filename,
+                timestamp=timezone.utcnow(),
                 stacktrace=stacktrace))
         session.commit()