graphics/qgis: Use default Python version

cmake's FindPython.cmake always returns the highest found Python version,
although the default version is needed here, since only for this usually
further tools like SIP etc. are installed.

Passing the path of the default version to Python_EXECUTABLE causes the
cmake configuration to use it.

Co-authored-by: Rainer Hurling <rhurlin@gwdg.de>

MFH: 2023Q3
(cherry picked from commit c3a413762d4f057b69e689df9ceeaf17a64419ec)