Allow the opendrr-api stack build to run without the need of GitHub personal access token

[anthonyfok]

Thanks to @tieganh et al. for completing and finalizing various parts of the project for publication, all relevant repos (OpenDRR/openquake-inputs, OpenDRR/earthquake-scenarios, OpenDRR/seismic-risk-model, etc.) are now public, or have been since 2022/2023.

While OpenDRR/CanadaSHM6 is private, it is used only for running OpenQuake calculations (e.g. creating new earthquake scenarios), which we don't do during an opendrr-api stack build.

It would be nice to make minor tweaks so that the python/build_exposure_ancillary.sh and python/add_data.sh can run to completion even if github_token is left empty in python/config.ini. May need to check some Python scripts too on this repo and OpenDRR/model-factory.

(But do keep the mechanism in case we need to work with private repos again, e.g. during development phase of the next major release.)