fix: build_sdist artifact name references undefined matrix variables

[gounthar]

Summary

In .github/workflows/build_wheels.yml, the build_sdist job uploads its artifact using:

name: cibw-wheels-${{ matrix.os }}-${{ strategy.job-index }}

But build_sdist has no strategy/matrix block. Both expressions expand to empty strings, producing the artifact name cibw-wheels--. The join_artifacts job's cibw-wheels-* pattern still matches it, but the name is misleading and fragile.

Fix

Change the artifact name to something stable, e.g.:

name: cibw-wheels-sdist

Notes

This was inherited via upstream sync. It does not block the current build, but it should be cleaned up to avoid confusion.

Spotted during review of #8.