adopt cache settings from

https://ectobit.com/blog/speed-up-github-actions-rust-pipelines/

.github/workflows/test.yml

@@ -42,9 +42,13 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-          key: cargo-build-${{ hashFiles('**/Cargo.lock') }}-${{ env.RUST_STABLE}}
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo-
 
       - name: cargo fmt
         uses: actions-rs/cargo@v1