Add caching for clippy step

Author: molpopgen

.github/workflows/tests.yml

@@ -76,6 +76,7 @@ jobs:
           profile: minimal
           toolchain: stable
           override: true
+      - uses: Swatinem/rust-cache@v1
       - run: rustup component add clippy
       - name: clippy (all targets)
         run: cargo clippy --all-targets --all-features -- -D warnings