How to include already existing paths to `nix-build --dry-run`'s output?

Hi. Some nixos projects (like mobile-nixos) do use unstable branch of nixpkgs, and so I wish to know how to estimate the binary cache coverage of a particular nixpkgs revision.

The shortest path I see is to run nix-build ... --dry-run and grep for “will be fetched” section. Unfortunately, the output doesn’t include paths which happen to exist on the particular machine.

How can I pretend that outputs don’t exist and have nix-build print out the full list of “will be fetched” / “will be built” (in order to compare it with the results of other nixpkgs revisions)?