.github/workflows: simplify and avoid external dependency

Github has a native tool to obtain the PR diff.

This also resolves a warning from the action log:

    2026-07-30T16:02:20.3415885Z Node 20 is being deprecated. This workflow is running with Node 24 by default. If you need to temporarily use Node 20, you can set the ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true environment variable. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>