Add actionlint config for self-hosted runner labels

actionlint rejects non-GitHub-hosted runner labels unless declared
in .github/actionlint.yaml. This was causing pre-commit.ci failures
on test-sdist.yml which uses linux-amd64-cpu8.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpcloud

.github/actionlint.yaml

@@ -0,0 +1,9 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# Self-hosted runner labels used in CI workflows.
+# Without this config, actionlint rejects non-GitHub-hosted labels.
+self-hosted-runner:
+  labels:
+    - linux-amd64-cpu8