Explicit pass the Python path to pipx in CI step

[abravalheri]

Summary of changes

I identified some recent problems in the CI:

creating virtual environment...
installing build...
* Creating isolated environment: venv+pip...
* Getting build dependencies for sdist...
/home/runner/work/setuptools/setuptools/_distutils_hack/__init__.py:53: UserWarning: Reliance on distutils from stdlib is deprecated. Users must rely on setuptools to provide the distutils module. Avoid importing distutils or import setuptools first, and avoid setting SETUPTOOLS_USE_DISTUTILS=stdlib. Register concerns at https://github.com/pypa/setuptools/issues/new?template=distutils-deprecation.yml
  warnings.warn(

Traceback (most recent call last):
  File "/opt/pipx/.cache/98ab2ca9c43e098/lib/python3.12/site-packages/pyproject_hooks/_impl.py", line 402, in _call_hook
    raise BackendUnavailable(
pyproject_hooks._impl.BackendUnavailable: Cannot import 'setuptools.build_meta'

ERROR Backend 'setuptools.build_meta' is not available.
Error: Process completed with exit code 1.

https://github.com/pypa/setuptools/actions/runs/12672452318/job/35316523998

Curiously the trace shows lib/python3.12/site-packages/pyproject_hooks/..., but the python that should have been used is Python 10. So there might be a problem with pipx picking up the wrong version of Python...

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
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