Python installation selection in Meson

I noticed that under meson, the selection of the Python installation
using the 'PYTHON' option doesn't work completely. The 'PYTHON' option
determined the Python binary that will be used to call the various build
support programs. But it doesn't affect the Python installation used
for PL/Python. For that, we need to pass the program determined by the
'PYTHON' option back into the find_installation() routine of the python
module. (Otherwise, find_installation() will just search for an
installation on its own.) See attached patch. I ran this through
Cirrus, seems to work.