CI: Add windows to pytest-nbval workflow

It'd be nice to add explicit testing on windows for the pytest-nbval CI workflows:

https://github.com/numpy/numpy-tutorials/blob/47fb909bbd7777f85d94a37647db4c4dd84f1bdc/.github/workflows/notebooks.yml#L17

I think this should mostly be a matter of adding conditional steps to the workflow where windows-specific CLI syntax is required.