Lint GitHub Actions workflows with actionlint

Statically checks workflow files (schema, expression contexts,
runner labels, shellcheck over run: scripts) on any PR or push
touching them. .github/actionlint.yaml declares the self-hosted
'admin' label. The existing workflows already pass cleanly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/actionlint.yaml

@@ -0,0 +1,4 @@
+self-hosted-runner:
+  # The admin node is registered as a self-hosted runner (see the infra repository)
+  labels:
+    - admin

.github/workflows/lint-actions.yml

@@ -0,0 +1,20 @@
+name: Lint GitHub Actions workflows
+
+on:
+  push:
+    branches: [main]
+    paths: ['.github/workflows/**', '.github/actionlint.yaml']
+  pull_request:
+    paths: ['.github/workflows/**', '.github/actionlint.yaml']
+
+jobs:
+  actionlint:
+    if: github.repository_owner == 'compiler-explorer'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Check workflow files
+        uses: docker://rhysd/actionlint:1.7.12
+        with:
+          args: -color