rpmbuild: properly cleanup after obtaining sources
    
    We currently don't clean a temporary directory used for obtaining
    sources, so we unintentionally spam /tmp. We don't clean even
    when the build succeeds, so this is not related to catching exceptions.
    
    The problem seems to be, that we are trying to clean in `__exit__`
    method, which is a mistake. The `__exit__` method executes when
    leaving a context - we don't use `*Provider` as a context.
    We should clean in destructor instead, hence `__del__`.
    
    Relates: #1245