ci: teach actionlint the ubuntu-26.04-arm runner label (#3375)

actionlint 1.7.12 does not recognize the ubuntu-26.04-arm GitHub-hosted
runner label yet (upstream: rhysd/actionlint#683). Declare it in
actionlint's config so the Lint GitHub Actions check passes for the
renovate runner bump (#3338). Remove once actionlint
ships built-in support for the label.

Author: setchy

.github/actionlint.yaml

@@ -0,0 +1,13 @@
+# WORKAROUND: actionlint does not recognize the `ubuntu-26.04-arm` GitHub-hosted
+# runner label yet. Upstream support is pending (rhysd/actionlint#683, duplicate #723),
+# so the pinned actionlint version used in .github/workflows/lint.yml would fail the
+# lint check with `label "ubuntu-26.04-arm" is unknown`.
+#
+# Declaring the label under `self-hosted-runner.labels` is actionlint's documented
+# mechanism for accepting runner labels it does not know about. Remove this entry
+# once the pinned actionlint version in .github/workflows/lint.yml ships built-in
+# support for `ubuntu-26.04-arm`; the label is a GitHub-hosted runner, not
+# self-hosted.
+self-hosted-runner:
+  labels:
+    - ubuntu-26.04-arm