In the test target of the makefile, secureboot is explicitly turned off for the new instance. When I try to run that, it will not do the secureboot provisioning and instead immediately boot IncusOS, and there won't be any mkosi key available inside the booted kernel, preventing incus-osd from running ("unsupported operating mode").
After I removed the secureboot setting everything worked fine.
But I saw in your stream that you are running it successfully with secureboot disabled, I don't quite understand how that works :D