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.

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