heptapod-ci: add `clippy` to the CI

This linter mostly makes our code more idiomatic, less surprising, has good
suggestions and catches bugs. It's widely used in the Rust community and now
part of the default toolchain when using `rustup`.

contrib/heptapod-ci.yml

@@ -41,6 +41,7 b' rust-cargo-test:'
     script:
         - echo "python used, $PYTHON"
         - make rust-tests
+        - make cargo-clippy
     variables:
         PYTHON: python3