Updates to CHANGELOG files should not trigger build/test

Currently we build and test on every PR, even if none of the files touched by the PR affect build or test. For example, changelog files.

Suggestion: skip build and test when the PR only affects changelog files. Note other forms of validation may still be applicable.

This will help speed up our regular releases as we can't start the release pipeline until the CHANGELOG.md has been updated, and running build and test are the slowest parts of that process.