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

• Make rules_pkg support raw symlinks feat: support raw symlinks from declare_symlink() bazelbuild/rules_pkg#930
• Make CI happy when its the default
• Make it the default for linux/mac
• Provide a windows equivalent