test: use process.env.PYTHON to spawn python

Co-authored-by: Jérémy Lal <kapouer@melix.org>
Fixes: https://github.com/nodejs/node/issues/34699

PR-URL: https://github.com/nodejs/node/pull/34700


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>