build/nexus: use hello_world for a model in order to fit in memory

On real hw we load the builtins bundle into a .5M carve-out of DRAM.
IREE models are too big to fit so include hello_world so there's something
for mltest to run.

Change-Id: I546a56926e50e01f5e883f268e4effe04f273b0f