Cache cargo-audit on CI

closes #3399

.github/workflows/ci.yaml

@@ -16,9 +16,22 @@ jobs:
       CARGO_NET_RETRY: 10
 
     steps:
+    - name: Install Rust toolchain
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: stable
+        profile: minimal
+        override: true
+
     - name: Checkout repository
       uses: actions/checkout@v2
 
+    - name: Cache cargo
+      uses: actions/cache@v1
+      with:
+        path: ~/.cargo
+        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+
     - run: cargo install cargo-audit
     - run: cargo audit