Run 'cargo audit' in CI

.github/workflows/rust.yml

@@ -3,6 +3,20 @@ name: electrs
 on: [push, pull_request]
 
 jobs:
+  audit:
+    name: Audit
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/install@v0.1
+        with:
+          crate: cargo-audit
+          version: 0.14.1
+          use-tool-cache: true
+      - uses: actions-rs/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+
   build:
     name: Build
     runs-on: ubuntu-latest