Tell actionlint that image ubuntu-24.04-arm is valid (#1136)

The addition of testing on ubuntu-24.04-arm causes `actionlint` to
produce a false positive error about the runner name being invalid. The
error happens because that particular runner image is one of the partner
images from https://github.com/actions/partner-runner-images and
apparently `actionlint` does not know the names of those by default.

This `actionlint` configuration file defines the runner name as valid.

Author: mhucka

.github/actionlint.yaml

@@ -0,0 +1,19 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+self-hosted-runner:
+  # We don't have self-hosted runners, but we do use some of the "partner"
+  # runner images at https://github.com/actions/partner-runner-images
+  labels:
+    - ubuntu-24.04-arm