Clear the cache when the pip/Pipenv version changes

[edmorley]

If the pip or Pipenv version changes we should clear the cache to:

1. Avoid any issues with backwards compatibility.
2. Help improve determinism between environments (eg between a staging app and a prod app, where the package manager version used to install a package might be different, if they were deployed at different times)

In particular, this blocks upgrading to pip 24.1 - since newer versions of pip aren't able to uninstall legacy Python packages with broken version specifiers. See:
#1619 (comment)