build,win: replace run-python subroutine with single find_python call

A subroutine does not work as a replacement for the `python` command
since one cannot use a subroutine call in a `for /F` loop.

PR-URL: https://github.com/nodejs/node/pull/18621


Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>