ci: add job to run cargo audit on pull_request and weekly

Author: notmandatory

.github/workflows/audit.yml

@@ -0,0 +1,19 @@
+name: Audit
+
+on:
+  pull_request:
+    paths:
+      - '**/Cargo.toml'
+      - '**/Cargo.lock'
+  schedule:
+    - cron: '0 0 * * 0' # Once per week
+
+jobs:
+
+  security_audit:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}