cmake/modules: always use the python3 specified in command line

cmake/modules: always use the python3 specified in command line

if another python3 with higher version is found by
find_package(Python3), the cmake's install script would just
install the python modules/extensions into that python3's
dist-package directory, and the packaging script would fail
to find these artifacts when trying to package them.

so we need to ensure that the install directories for python
modeules/extensions are always "versioned" with WITH_PYTHON3
cmake option.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>