I think it would be better to update talos_robot/talos_description/test/CMakeLists.txt which needs rbdl. I believe that if you add the path to FindRBDL.cmake in the following, you will be able to find the .so.
list(APPEND CMAKE_MODULE_PATH "Directory where FindRBDL.cmake is located")

It is possible that you may need to change the rbdl in find_package to RBDL.
By the way, this is not the main topic, but it looks like the only thing that needs rbdl is the test, and I think I can get it to work by deleting talos_robot/talos_description/test/.