CI: use pipx and build to simplify and remove a workaround
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry


This cleans up the SDist build slightly. First, it uses PyPA's "build" instead of directly calling `setup.py`, which will respect PEP 518 requirements and avoids the workaround of installing NumPy to be able to run setup.py. It also uses pipx to avoid needless setup; PyPA's pipx is provides as a first-class package manager on GitHub Actions (and Azure, they share images).
