fix: fix add_target_repo when signing role is the top-level targets role

[renatav]

Description (e.g. "Related to ...", etc.)

Role paths should not be added if the signing role is the top-level targets role

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)