Respect --skip-build flag in setup.py

[hoodmane]

python setup.py bdist-wheel --skip-build is supposed to skip building the package. We need this in Pyodide https://github.com/pyodide/pyodide/pull/2027 because of the way our cross compilation works. I'm actually not sure why this isn't causing trouble for us on our main branch,

[Midnighter]

@hoodmane thank you for the change suggestion. Could you rebase your work onto the latest state on the master branch, please? I don't see an issue with introducing this code otherwise. Just want the tests to pass.