Cannot link code against RDataFrame if builtin Vdt is enabled

When ROOT has builtin_vdt=ON, and a user wants to link against RDataFrame after installation, the Vdt::Vdt target (imported target created by ROOT) cannot be found and linking fails. The problem is that when Vdt is builtin to ROOT, the target for it cannot be exported, as it would point to the build directory.

The symptom can be seen here: https://root-forum.cern.ch/t/cmake-linking-rdataframe/30944/8.