Build: Move spider-check from Travis cron to GitHub cron

Previously the script was in the 'spider-check' branch, which
is how Travis crons were configured. For GitHub, the script
is required to be in the main branch.

Ref jquery/infrastructure#455.

Author: Krinkle

.github/workflows/spider-check.yaml

@@ -0,0 +1,28 @@
+name: spider-check
+on:
+  # Once a week on Thursday at 11:30 AM UTC
+  schedule:
+    - cron: '30 11 * * 4'
+  # Or manually
+  workflow_dispatch:
+  # Or when developing this workflow
+  push:
+    paths:
+      - .github/workflows/spider-check.yaml
+  pull_request:
+    paths:
+      - .github/workflows/spider-check.yaml
+
+jobs:
+  spider-check:
+    # Includes Python 3
+    # https://github.com/actions/virtual-environments/blob/ubuntu20/20210318.0/images/linux/Ubuntu2004-README.md
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          repository: jquery/hydra-link-checker
+          ref: v1.0.2
+
+      - name: Run hydra-link-checker
+        run: python3 hydra.py "https://releases.jquery.com/"