ci: determine which tests to run by diffing with previous builds

Created by: asdine

This PR determines which tests to run by running a diff with the previous successful build commit. If there is no previous passing build or if the diff fails for any reason, if runs a diff with main.

⚠️ Before merging: Make sure the BUILDKITE_API_TOKEN is present in buildkite

📖 Notes: The mechanism to extract the previous commit from the previous build is not perfect because sometimes Buildkite seems to not be able to find the previous commit in the history. See comment

Fixes #25164 (closed)