Support non-bash based script bootstrap

Right now, the stage1 script bootstrap requires bash. It has a variety of bashisms or non-posix constructs that prevent it from working in other shells.

Bash isn't bad, it is just a heavier weight shell than e.g. dash (default on ubuntu/debian).

Looking at the code most changes are minor. The major change is the usage of arrays -- posix doesn't actually specify arrays.

Related: https://github.com/bazel-contrib/rules_python/issues/2500

Start by locating the stage1 script bootstrap and reviewing its bash-specific constructs, especially the array usage. Test the bootstrap with dash, the non-bash shell called out in the issue, and confirm that it completes successfully without changing its existing behavior.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.