feat(core): use a shell script bootstrap by default

Fixes #2521
Fixes #2983

Author: aignas

CHANGELOG.md

@@ -70,7 +70,8 @@ END_UNRELEASED_TEMPLATE
 * (gazelle) Switched back to smacker/go-tree-sitter, fixing
   [#2630](https://github.com/bazel-contrib/rules_python/issues/2630)
 * (ci) We are now testing on Ubuntu 22.04 for RBE and non-RBE configurations.
-* (core) #!/usr/bin/env bash is now used as a shebang in the stage1 bootstrap template.
+* (core) `#!/usr/bin/env bash` is now used as a shebang in the stage1 bootstrap template.
+* (core) the flag `bootstrap_impl=script` has been switched for UNIX platforms.
 
 [20250723]: https://github.com/astral-sh/python-build-standalone/releases/tag/20250723
 

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,