document `--debug` for build-locally.py

- [x] I read [the conda-forge documentation](https://conda-forge.org/docs/user/introduction.html#how-can-i-install-packages-from-conda-forge) and could not find the solution for my problem there.

Issue: `BUILD_WITH_CONDA_DEBUG` is not mentioned in the conda-forge documentation. It would be nice to mention it when `build-locally.py` is mentioned (https://conda-forge.org/docs/maintainer/updating_pkgs.html#testing-changes-locally)