rust: checksum: Use BTreeMap instead of HashMap

By using `BTreeMap`, we force the checksum files to be ordered.

This prevents excessive noise during `./crate_tool regenerate --all`

Bug: 388273315
Test: ./crate_tool regenerate aarch64-paging , temp commit, run again, check no diffs.
Change-Id: Ia1de8a43723329f9ac56fde16f7be6005a449bdd
1 file changed