Disable GitHub Actions Caching Across CI Workflows

[harryswift01]

Summary

This PR will disable all explicit caching in the GitHub Actions workflows to ensure every CI and regression run executes in a completely fresh environment.

Changes

Remove pip caching from setup-python

• Removed cache: pip from all actions/setup-python steps.
• Ensures Python dependencies are freshly installed on every workflow run.
• Prevents reuse of cached wheels and virtual environment artifacts between runs.

Remove regression test data caching

• Removed the actions/cache@v4 step for .testdata.
• Ensures regression test data is regenerated or freshly downloaded every run.
• Prevents stale regression data from persisting between CI executions.

Impact

• CI runs will take longer due to fresh dependency installation.
• Regression workflows will no longer reuse cached .testdata.
• Workflow behaviour becomes more deterministic and reproducible.
• Eliminates potential issues caused by stale or corrupted caches.

[jimboid]

Removing the caching is potentially a fix for the random test failure behaviour seen with previous CI runs. Although the pip caching was probably safe and the cause origin being the test/data caching, this is a great first pass at solving the intermittent issue of tests failing on github but not locally.