Make script bootstrap the default

This is a general tracking issue for making the default bootstrap method the "script" implementatation (i.e. `--bootstrap_impl=script`).

The current default is "system_python", which has the problem that it relies on the system having some Python already installed and available via shebang in order to run a py_binary (which often has its own interpreter).

TODO

* [x] Make rules_pkg support raw symlinks https://github.com/bazelbuild/rules_pkg/pull/930
* [x] Make CI happy when its the default
* [ ] Make it the default for linux/mac
* This is blocked by: Bazel 8+ is required for rules_pkg to package a py_binary when build-time relative symlinks are enabled (`--venvs_use_declare_symlink=yes`, the default).
* [ ] Provide a windows equivalent