bump actions/cache version from 2 to 4

[cibere]

The python workflow in #477 is failing due to the following error:

Error: This request has been automatically failed because it uses a deprecated version of `actions/cache: v2`. Please update your workflow to use v3/v4 of actions/cache to avoid interruptions. Learn more: https://github.blog/changelog/2024-12-05-notice-of-upcoming-releases-and-breaking-changes-for-github-actions/#actions-cache-v1-v2-and-actions-toolkit-cache-package-closing-down

this PR simply bumps the cache version from 2 to 4 to fix this.

I haven't tested it, but from what I can tell, it should still work properly