Refactor ExternalDagLink to not create ad hoc TaskInstances

This operator link creates an ad hoc TaskInstance to build its output URL. Generally we moved away from this approach starting with apache#21285.

This PR updates the operator link to use `TaskInstance.get_task_instance()` instead.

airflow/sensors/external_task.py

@@ -45,6 +45,8 @@
 if TYPE_CHECKING:
     from sqlalchemy.orm import Query, Session
 
+    from airflow.models.baseoperator import BaseOperator
+    from airflow.models.taskinstancekey import TaskInstanceKey
     from airflow.utils.context import Context
 
 
@@ -57,10 +59,20 @@ class ExternalDagLink(BaseOperatorLink):
 
     name = "External DAG"
 
-    def get_link(self, operator, dttm):
-        ti = TaskInstance(task=operator, execution_date=dttm)
+    def get_link(self, operator: BaseOperator, *, ti_key: TaskInstanceKey) -> str:
+        ti = TaskInstance.get_task_instance(
+            dag_id=ti_key.dag_id, run_id=ti_key.run_id, task_id=ti_key.task_id, map_index=ti_key.map_index
+        )
+
+        if TYPE_CHECKING:
+            assert ti is not None
+
         operator.render_template_fields(ti.get_template_context())
-        query = {"dag_id": operator.external_dag_id, "execution_date": dttm.isoformat()}
+        query = {
+            "dag_id": operator.external_dag_id,  # type: ignore[attr-defined]
+            "execution_date": ti.execution_date.isoformat(),  # type: ignore[union-attr]
+        }
+
         return build_airflow_url_with_query(query)