graphics/qgis-ltr: 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>

PR: 272457
MFH: 2023Q3