Improve automated config validation in CI

[a-wai]

Currently, the YAML config files are processed for basic validation using the tests/validate_yaml.py script. However, this script doesn't catch all issues; in particular, it doesn't check the sorting of top-level keys, which should be more strictly enforced as it improves long-term maintainability.

It would therefore be useful to create an additional GH Action, which would run kci config validate on the changed config. This requires of course having kci available in the CI environment, meaning it should likely run on the kernelci/kernelci docker image.