Better support for cibuildwheel config in `pyproject.toml`

[ConorMacBride]

Currently the two publish workflows create jobs which set CIBW_BUILD and CIBW_ARCHS environment variables depending on the list of targets specified as an input to this workflow. These environment variables override any cibuildwheel config for build or archs set in pyproject.toml. (It should at least be documented that the publish workflows do not support build or archs within pyproject.toml.)

A fix would be to update tools/load_build_targets.py to parse pyproject.toml for build and archs configuration, and merge/adjust/replace the targets specified as an input to this workflow with the pyproject.toml config. I think the simplest solution would be to support either build config in pyproject.toml OR the targets workflow input.

[Cadair]

Could we "just" make targets optional? Then we wouldn't have to parse pyproject and just let cibuildwheel handle it?