Nanoid construction is not determinisitc

[john-h-kastner-aws]

Selecting EntityUIDGenMode::Nanoid option makes our generators nondeterministic because the nanoid!() macro randomly constructs an id without using a seed taken from the unstructured byte array. This isn't a major issue since it's currently only used by the cedar-policy-generators CLI, but it's a foot-gun for anyone who wants to depend on the generators as a library.