Add cargo-audit to pull requests (model-checking#1110)

.github/workflows/audit.yml

@@ -0,0 +1,18 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+# A temporary, non-required workflow to notify us when cargo-audit has things to say.
+# TODO: Remove this, and make cargo-audit part of release automation (when we have it) instead.
+# And maybe also figure out why dependabot wasn't notifying us. :(
+
+name: Cargo Audit
+on: [pull_request]
+
+jobs:
+  audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}