Currently, there's a hack at https://github.com/tox-dev/workflow/blob/main/.github/workflows/tox.yml#L174-L178. It can be solved by $sbj which is why I've originally requested it upstream @ actions/setup-python#309, and it got implemented via actions/setup-python#405. The first version supporting it is v4.0.0.

Ref: https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#python-path.