Lint GitHub Actions workflows with actionlint

Part of an org-wide rollout: statically checks workflow files
(schema, expression contexts, runner labels, shellcheck over
run: scripts) on any push/PR touching them.
Bumped actions/checkout v2 -> v4 (outdated runner).

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

Author: mattgodbolt

.github/actionlint.yaml

@@ -0,0 +1,18 @@
+self-hosted-runner:
+  # Labels of Compiler Explorer's self-hosted runners (see the ce-ci repository)
+  labels:
+    - ce
+    - small
+    - medium
+    - admin
+    - lin-builder
+    - win-builder
+
+# Enforce shellcheck errors and warnings in embedded run: scripts, but not
+# info/style — consistent with the org-wide actionlint rollout.
+paths:
+  .github/workflows/**/*.yml:
+    ignore: &shellcheck-info-style
+      - 'shellcheck reported issue in this script: SC\d+:(info|style):'
+  .github/workflows/**/*.yaml:
+    ignore: *shellcheck-info-style

.github/workflows/build-test.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: windows-2022
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Add msbuild to PATH
         uses: microsoft/setup-msbuild@v1.1

.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