fix(campaign): honor acceptance temp selection

dev/mail-testbed/run_celery_redelivery_acceptance.py

@@ -587,6 +587,12 @@ def _runtime_module_versions(registry: Any) -> dict[str, str]:
     return versions
 
 
+def _create_runtime_root() -> Path:
+    """Create the isolated runtime under the platform-selected temp root."""
+
+    return Path(tempfile.mkdtemp(prefix="govoplan-campaign-celery-redelivery-"))
+
+
 def _bootstrap_and_run(
     *,
     settings: TestbedSettings,
@@ -594,9 +600,7 @@ def _bootstrap_and_run(
     redis_url: str,
     visibility_timeout_seconds: int,
 ) -> dict[str, Any]:
-    runtime_root = Path(
-        tempfile.mkdtemp(prefix="govoplan-campaign-celery-redelivery-", dir="/tmp")
-    )
+    runtime_root = _create_runtime_root()
     database = None
     try:
         os.environ.update(