ci: fix audit step

This was failing in the 2021 edition. See the discussion in TrueLayer/ginepro#25

Author: marcoieni

.github/workflows/audit.yml

@@ -16,6 +16,13 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
+      # Ensure that the latest version of Cargo is installed
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          profile: minimal
+          override: true
       - uses: actions-rs/audit-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}