Just use an empty dict. It's a way to resolve, e.g., "virtuals:unix" in the build spec to "unix/abasdf234234234", where the latter is a build artifact that you do NOT want to enter the hash (because, e.g., `cp`, `mv` etc. are assumed to always be the same).

The virtuals systems needs some improvement anyway, so don't worry about it, just pass an empty dict.