Use Correct name for CMake Python_EXECUTABLE variable.
Use Correct name for CMake Python_EXECUTABLE variable.

nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,hsorby/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,nickerso/libcellml