Add CI workflow to validate GitHub Actions workflows

On every push or pull request that affects the repository's GitHub Actions workflows, and periodically, validate them
against the JSON schema.

Author: per1234

.github/workflows/check-workflows-task.yml

@@ -0,0 +1,35 @@
+# Source: https://github.com/arduino/tooling-project-assets/blob/master/workflow-templates/check-workflows-task.md
+name: Check Workflows
+
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/*.ya?ml"
+      - "Taskfile.ya?ml"
+  pull_request:
+    paths:
+      - ".github/workflows/*.ya?ml"
+      - "Taskfile.ya?ml"
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch breakage resulting from changes to the JSON schema.
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Install Task
+        uses: arduino/setup-task@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          version: 3.x
+
+      - name: Validate workflows
+        run: task --silent ci:validate

Taskfile.yml

@@ -2,6 +2,32 @@
 version: "3"
 
 tasks:
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/check-workflows-task/Taskfile.yml
+  ci:validate:
+    desc: Validate GitHub Actions workflows against their JSON schema
+    vars:
+      # Source: https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/github-workflow.json
+      WORKFLOW_SCHEMA_URL: https://json.schemastore.org/github-workflow
+      WORKFLOW_SCHEMA_PATH:
+        sh: mktemp -t workflow-schema-XXXXXXXXXX.json
+      WORKFLOWS_DATA_PATH: "./.github/workflows/*.{yml,yaml}"
+    cmds:
+      - |
+        wget \
+          --quiet \
+          --output-document="{{.WORKFLOW_SCHEMA_PATH}}" \
+          {{.WORKFLOW_SCHEMA_URL}}
+      - |
+        npx \
+          --package=ajv-cli \
+          --package=ajv-formats \
+          ajv validate \
+            --all-errors \
+            --strict=false \
+            -c ajv-formats \
+            -s "{{.WORKFLOW_SCHEMA_PATH}}" \
+            -d "{{.WORKFLOWS_DATA_PATH}}"
+
   # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/check-general-formatting-task/Taskfile.yml
   general:check-formatting:
     desc: Check basic formatting style of all files