Rollup merge of #148636 - xSetech:boostrap/set-python-on-macos, r=jieyouxu

bootstrap: respect `build.python` on macOS

The `python()` method was hardcoded to return `/usr/bin/python3` on macOS, ignoring the `build.python` config option. This change respects the config while maintaining the system Python as the default.

Author: Zalathar