Introduce CI workflow running cargo audit

In order to continuously monitor our dependencies for security
vulnerabilities, we introduce a new CI job that will use `cargo audit`
to check for any known vulnerabilities.

This job is run on a daily schedule. For each new advisory, a new issue
will be created.

.github/workflows/audit.yml

@@ -0,0 +1,19 @@
+name: Security audit
+on:
+  schedule:
+    - cron: '0 0 * * *'
+
+jobs:
+  audit:
+    strategy:
+      matrix:
+        platform: [
+          ubuntu-latest,
+          macos-latest,
+          ]
+    runs-on: ${{ matrix.platform }}
+    steps:
+      - uses: actions/checkout@v3
+      - uses: rustsec/[email protected]
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}