Improve automated config validation in CI

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.