Provide some way of building against multiple manylinux images

cibuildwheel has a default build image for manylinux wheels, currently it's `manylinux2010`. There are a number of env vars which allow you to specify which image to use (https://cibuildwheel.readthedocs.io/en/stable/options/#manylinux-image) but building multiple of them would involve two jobs with different values I think.