Actions do not upload source release

[tammojan]

The new scikit-build-wheel based build system now automatically uploads wheels for new releases to pypi. We forgot that it also should upload a source release.

(I created a source release for 3.6.1 manually from the tag and uploaded it to pypi.)

[gmloose]

We did not forget, but it somehow failed to do so. We do build a tar ball, and it should have been uploaded as part of the upload_pypi job in .github/workflows/build_release.yml. Or is it maybe related to the fact that the publish action by the CI pipeline failed the first time due to missing credentials?