feat: default to bootstrap script for non-windows

CHANGELOG.md

@@ -55,6 +55,7 @@ END_UNRELEASED_TEMPLATE
 {#v0-0-0-changed}
 ### Changed
 
+* (rules) {obj}`--bootstrap_impl=script` is the default for non-Windows.
 * (rules) On Windows, {obj}`--bootstrap_impl=system_python` is forced. This
   allows setting `--bootstrap_impl=script` in bazelrc for mixed-platform
   environments.

docs/api/rules_python/python/config_settings/index.md

@@ -233,6 +233,10 @@ Values:
 ::::{bzl:flag} bootstrap_impl
 Determine how programs implement their startup process.
 
+The default for this depends on the platform:
+* Windows: `system_python` (**always** used)
+* Other: `script`
+
 Values:
 * `system_python`: Use a bootstrap that requires a system Python available
   in order to start programs. This requires
@@ -257,6 +261,11 @@ instead.
 :::{versionadded} 0.33.0
 :::
 
+:::{versionchanged} VERSION_NEXT_FEATURE
+* The default for non-Windows changed from `system_python` to `script`.
+* On Windows, the value is forced to `system_python`.
+:::
+
 ::::
 
 ::::{bzl:flag} current_config

python/config_settings/BUILD.bazel

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