build,win: fix Python detection on localized OS

This code previously assumed the default value was always printed to
the console by reg.exe as "(default)", but this is not true on
localized versions of Windows and can contain spaces.

Fixes: https://github.com/nodejs/node/issues/29417

PR-URL: https://github.com/nodejs/node/pull/29423


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: David Carlier <devnexen@gmail.com>