Don't format generated code from `gentest` and `eest make test` if `ruff` is not present

For developers, who have cloned `ethereum/execution-specs`, generated code should be formatted by `ruff` using the settings from `pyproject.toml`. Then's it ready to check-in and passes checks.

However, for package users, to avoid adding `ruff` as a dependency package, it seems fine to simply skip this step, especially since this means we can skip adding `ruff` as a package dependency.

As part of this work, we should remove the addition of the `lint` dependency group from `tests_pytest_pypy3` tox env:
https://github.com/ethereum/execution-specs/blob/38c72986cb7b67fa41ea38d393cc241b25206e5c/tox.ini#L32-L34