build: try “python3” as a last resort for 3.x

So that Xcode's Python 3 gets picked up.

PR-URL: https://github.com/nodejs/node/pull/35983


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>