build,win: fix python detection script

Handle spaces in the path to python.exe, in case it is installed
under some directory like "C:\Program Files".

PR-URL: https://github.com/nodejs/node/pull/14546


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>