In case that might help, I found a workaround for a similar problem, looking at the cmake doc : https://cmake.org/cmake/help/v3.0/module/FindPythonLibs.html
You must set two env vars for cmake to find coherent versions. Unfortunately this is not a generic solution...
cmake -DPYTHON_LIBRARY=${HOME}/.pyenv/versions/3.8.0/lib/libpython3.8.a -DPYTHON_INCLUDE_DIR=${HOME}/.pyenv/versions/3.8.0/include/python3.8/ cern_root/