fix: add missing config-file option to the action

[henryiii]

This is missing, but might be handy if users want to use config files other than pyproject.toml.

[joerick]

It would be nice if we didn't have to specify the default here too, from a DRY perspective.

ahhhh, I think this default might present an issue, actually. If a config file is specified, then cibuildwheel requires it to exist. If it's not set, then the Options class will use pyproject.toml, if it's present. So this wouldn't work for projects without a pyproject.toml.

Maybe we'd have to use some bash to remove the argument if it's not set in the action? Or we could change Options to interpret config_file="" as 'not set'.

[henryiii]

Ahh, yes, I thought of that before, and then forgot about it.

I was thinking --config-file="" would be an option to turn off config file discovery completely; if you want to ignore the config in pyproject.toml, this would be a handy shortcut, possibly?

[henryiii]

That would then also provide different behavior for this option, too, and we haven't supported empty strings here in the past - I've changed this to config_file="" as 'not set'. Someone who wanted to override the pyproject.toml and didn't want a new toml file can just point this at an empty file, I suppose.