fix(demand): force mysql run output isolation

examples/demand/run_existing_execution_mysql.py

@@ -34,9 +34,9 @@ def _safe_path_segment(value: str) -> str:
 def _prepare_run_output_paths(execution_id: int, run_label: str) -> None:
     run_root = Path(__file__).parent / "test_output_data" / "mysql_runs" / _safe_path_segment(run_label)
     result_base_dir = run_root / "intermediate" / "result"
-    os.environ.setdefault("DEMAND_RESULT_BASE_DIR", str(result_base_dir))
-    os.environ.setdefault("DEMAND_OUTPUT_BASE_DIR", str(run_root / "output"))
-    os.environ.setdefault("DEMAND_TRACE_STORE_PATH", str(run_root / ".trace"))
+    os.environ["DEMAND_RESULT_BASE_DIR"] = str(result_base_dir)
+    os.environ["DEMAND_OUTPUT_BASE_DIR"] = str(run_root / "output")
+    os.environ["DEMAND_TRACE_STORE_PATH"] = str(run_root / ".trace")
 
     demand_items_path = result_base_dir / str(execution_id) / f"execution_id_{execution_id}_demand_items.json"
     if demand_items_path.exists():