Make the swig generation script use the correct python executable.

It was hardcoding the value "python", which will end up at best
getting a different python executable (if the user has overridden
the value of PYTHON_EXECUTABLE), and at worst encountering an
error (if there is no copy of python on the system path).

This patch changes the script to use sys.executable so that it
runs the sub-script with the same executable that it was run with.