Use `sort_key` attribute for sorting checks in `--display-checks`
    
    Previously, errors such as
    
    TypeError: '<' not supported between instances of 'RCheckCheckMacro' and 'RCheckBuildRequires'
    
    would result from trying to sort the check classes directly, which don't implement the `__lt__` method for sorting.
    
    This fix uses the `sort_key` attribute inherited from `AbstractCheck` to sort the checks.
    
     * src/FedoraReview/review_helper.py: Use `sort_key` attribute for sorting checks