Add rust-cache

Author: tjtelan

.github/workflows/release-plz.yml

@@ -55,6 +55,7 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}
+      - uses: Swatinem/rust-cache@v2
       - name: Install cargo-msrv and update MSRV in Cargo.toml
         timeout-minutes: 10
         run: |