ci/github-actions: add audit

* For now auditing should be done daily and on each push to main.
* I have used rustsec's fork because it is more featureful and it has
  some fixes done. Hopefully it should just work.

.github/workflows/audit.yml

@@ -0,0 +1,17 @@
+name: Audit for RUSTSEC advisories
+
+on:
+  push:
+    branches: main
+  schedule:
+    - cron: '0 0 * * *'
+
+jobs:
+  audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: action/checkout@v1
+      - uses: rustsec/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+