ci: Convert audit job to (scheduled) GitHub actions workflow

.github/workflows/audit.yml

@@ -0,0 +1,12 @@
+on:
+  schedule:
+    # every monday at 4AM (UTC?)
+    - cron: '0 4 * * 1'
+jobs:
+  audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions-rs/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}