Make script bootstrap the default

[rickeylev]

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

[AustinSchuh]

FYI, I tested this out and just hit https://github.com/bazel-contrib/rules_python/issues/3187 .

[FrankPortman]

I have found a decent enough workaround but should https://github.com/bazel-contrib/rules_python/issues/3324 be a sub issue to this?