ci: try out cargo caching

.github/workflows/ci.yml

@@ -120,6 +120,8 @@ jobs:
               run: |
                   bin/nix-build-and-cache ci
                 
+            - uses: Swatinem/rust-cache@v2
+
             - name: Run CI tests
               run: |
                   direnv exec . engage > >(tee -a test_output.log)