FindPyUblas.cmake: fix Python executable.

Since Python 3 is the default Python version on some distributions (e.g. Arch Linux), we need to use the CMake variable storing the actual Python executable. Then, to use Python 2, since CMake often mixes up Python 2/3 libraries (even if Python 2 is set in `CMakeLists.txt`):

```sh
cmake .. -DPYTHON_EXECUTABLE=/usr/bin/python2 \
         -DPYTHON_INCLUDE_DIR=/usr/include/python2.7 \
         -DPYTHON_LIBRARY=/usr/lib/libpython2.7.so
```