GHSA-j8x2-777p-23fc: tough cyclic delegation graphs are not detected

In a TUF repository, the targets role’s signature indicates which target files are trusted by clients. The role can delegate full or partial trust to other roles, meaning that that role is trusted to sign target file metadata. Delegated roles can further delegate trust to other delegated roles. When searching for metadata about a given target, tough failed to detect cyclical role delegations.