meson: Improve/fix Python version selection

When we look for the Python installation using the meson python
module, we should make it use the python program previously determined
by the 'PYTHON' option.  Otherwise, it will just use its own search
and the 'PYTHON' option won't affect it.  We need this to be able to
select the Python installation to build PL/Python against.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/3f5427e8-cef6-474d-ae88-13db1bfdc1a7%40eisentraut.org

meson.build

@@ -1055,7 +1055,7 @@ pyopt = get_option('plpython')
 python3_dep = not_found_dep
 if not pyopt.disabled()
   pm = import('python')
-  python3_inst = pm.find_installation(required: pyopt)
+  python3_inst = pm.find_installation(python.path(), required: pyopt)
   if python3_inst.found()
     python3_dep = python3_inst.dependency(embed: true, required: pyopt)
     # Remove this check after we depend on Meson >= 1.1.0