add github action build cache

.github/workflows/rust.yml

@@ -14,6 +14,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable