build: workaround for gclient python3 issues

gclient doesn't support Python 3 yet. To workaround that problem, add an
enviroment variable to override the Python version used by ./configure.

Signed-off-by: Matheus Marchini <mmarchini@netflix.com>

PR-URL: https://github.com/nodejs/node/pull/32140


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>