chore(ci): cache cargo dependencies for test job

.github/workflows/ci.yml

@@ -79,6 +79,17 @@ jobs:
           toolchain: stable
           override: true
 
+      - name: Cache Cargo dependencies
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ${{ runner.os }}-cargo-${{ hashFiles('Cargo.lock') }}
+
       - name: Setup cargo-tarpaulin
         run: |
           curl -s https://api.github.com/repos/xd009642/tarpaulin/releases/latest | \