Use `setup-python` just for caching

Description:
It would be good to be able to use this action just to cache dependencies, without also downloading Python. For example:

- uses: actions/setup-python@v5
  with:
    cache: pip

This currently doesn't work, because the version is not set.

You can do something similar with setup-node, for example:

- uses: actions/setup-node@v4
  with:
    cache: npm

Justification:
It would be able to use the existing logic in this action to cache dependencies, instead of having to use the cache action manually.

Are you willing to submit a PR?
Yes

まず、action のキャッシュ処理とバージョン検証を確認し、次に issue で説明されている setup-node の動作と比較します。cache: pip を使用する workflow で Python のバージョンを設定せずに済み、その構成を対象とするテストがある状態になれば完了です。