Only run pyproject.toml lint rules when enabled (#5578)
## Summary
I was testing some changes on Airflow, and I realized that we _always_
run the `pyproject.toml` validation rules, even if they're not enabled.
This PR gates them behind the appropriate enablement flags.
## Test Plan
- Ran: `cargo run -p ruff_cli -- check ../airflow -n`. Verified that no
RUF200 violations were raised.
- Run: `cargo run -p ruff_cli -- check ../airflow -n --select RUF200`.
Verified that two RUF200 violations were raised.
