cmake: fix finding python2, and check that it is really 2 python version check removed in a4d044c0cd97f68b4b3dde414b83d609719d33d7 ("cmake: use PYTHON_EXECUTABLE to find python2"), but in bcb990ab7cf4921c2c30a1aa4fcb4c0131fffe94 ("cmake/win32: fix running regress, but fixing finding python2 interpreter") we set PYTHON_EXECUTABLE only if it exists.