[python] Correctly specify the pip cache in the Github workflow

[thetorpedodog]

Every single run of the upload-to-pypi task, I would see:

Post job cleanup.
Error: Unexpected end of JSON input

and I had no idea why. But looking at the setup-python step:

cache: pip
cache-dependency-path: python-spec/requirements-py.txt
check-latest: false
token: ***
update-environment: true

There is no requirements-py.txt file, because there is no matrix to get a matrix.python-version from, because it's not a matrixed step.