The default on macOS is clarified in bootstrap.example.toml

Author: xSetech

bootstrap.example.toml

@@ -322,7 +322,9 @@
 # Python interpreter to use for various tasks throughout the build, notably
 # rustdoc tests, the lldb python interpreter, and some dist bits and pieces.
 #
-# Defaults to the Python interpreter used to execute x.py.
+# Defaults to the Python interpreter used to execute x.py, or /usr/bin/python3
+# on macOS. Note that LLDB tests require a Python version compatible with the
+# LLDB plugin to be loaded.
 #build.python = "python"
 
 # The path to (or name of) the resource compiler executable to use on Windows.