🔧 use separate build directories per Python version

[burgholzer]

Description

Revert to using separate build directories for the Python builds and rather rely on the caching performance of ccache between builds.

Checklist:

• [x] The pull request only contains commits that are related to it.
• [x] I have added appropriate tests and documentation.
• [x] I have made sure that all CI jobs on GitHub pass.
• [x] The pull request introduces no new warnings and follows the project's style guidelines.