Produced zipapp is not reproducible due to `_bootstrap` file ordering

[timothyg-stripe]

👋 We are using shiv in reproducible mode, but we found that the output is not idempotent due to this block of code that adds the _bootstrap directory:

shiv/src/shiv/builder.py

Lines 165 to 177 in a353d10

	bootstrap_target = Path("_bootstrap")
	for path, name in iter_package_files(bootstrap):
	data = path.read_bytes()
	write_to_zipapp(
	archive,
	str(bootstrap_target / name),
	data,
	zipinfo_datetime,
	compression,
	stat=path.stat(),
	)

In particular, iter_package_files(bootstrap) (which eventually calls Path.iterdir()) is not guaranteed to be in any particular order.