feat(ci): add audit workflow

This runs `cargo deny check` every night.

Signed-off-by: Sandro-Alessio Gierens <sandro@gierens.de>

.github/workflows/audit.yml

@@ -0,0 +1,16 @@
+name: Security audit
+on:
+  schedule:
+    - cron: '0 0 * * *'
+  push:
+    paths:
+      - '**/Cargo.toml'
+      - '**/Cargo.lock'
+jobs:
+  security_audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: taiki-e/install-action@cargo-deny
+      - name: Scan for vulnerabilities
+        run: cargo deny check advisories