revert: change default bootstrap back to system_python

Author: rickeylev

python/config_settings/BUILD.bazel

@@ -90,7 +90,7 @@ string_flag(
 
 rp_string_flag(
     name = "bootstrap_impl",
-    build_setting_default = BootstrapImplFlag.SCRIPT,
+    build_setting_default = BootstrapImplFlag.SYSTEM_PYTHON,
     override = select({
         # Windows doesn't yet support bootstrap=script, so force disable it
         ":_is_windows": BootstrapImplFlag.SYSTEM_PYTHON,