Update build-sdist job

[Michael-J-Ward]

Follow on from #829

The Issue

build-sdist does not generate any artifact.

The actions/upload-artifact step is looking to archive files in target/wheels/*, which sdist builds do not generate.

The generated archive from the logs (reference job run):

Built source distribution to /home/runner/work/datafusion-python/datafusion-python/dist/datafusion-40.0.0.tar.gz

Context

The release documentation currently expects only wheels to be included in dist.zip and for the sdist to be generated manually.

datafusion-python/dev/release/README.md

Lines 142 to 155 in 805183b

	#### Publish Python Source Distribution to testpypi
	Download the source tarball created in the previous step, untar it, and run:
	```bash
	maturin sdist
	```
	This will create a file named `dist/datafusion-0.7.0.tar.gz`. Upload this to testpypi:
	```bash
	python3 -m twine upload --repository testpypi dist/datafusion-0.7.0.tar.gz
	```

Options

• Update the step to upload the tar archive as an artifact and update the release docs to use that.
• Update the step to assert that no wheels are generated in target/wheels/*, which will verify the sdist build and ensure the behavior doesn't change under our feet.
• Remove the step altogether.