Revert "build: use `find_package(Python3)` if available"

This reverts commit cd84bfb8142bc7ff3a07a188ffb809f1d86d1fd7.  Although
this passed the CI in phabricator, some of the bots are missing python3
packages, revert it temporarily.

GitOrigin-RevId: be884b79352de8917ad17fdb9bd70ec982ca8bdd
6 files changed