Refresh the PR cache of the parent repo rather than always the current one
    
    If the current project is a fork, we want to refresh the cache of the main
    project and otherwise, we fall back to refreshing the cache of the current
    project.
    
    Fixes https://pagure.io/pagure/issue/2940
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>