fix: build expert on latest nixpkgs

[idabmat]

Fixes #403
Picking up on the changes from NixOS/nixpkgs#475224

I don't know if there are more changes that @adamcstephens wants to
include. But at least it now builds on latest nixpkgs.

I've refrained from updating the flake.lock to avoid any unforeseen
changes though. Let me know if I should bump it.

[adamcstephens]

I started working on this the other day, but got sidetracked going a bit deeper on some other improvements that @mhanberg and I were discussing on discord.

The flake.lock is pretty old in this repo. I'd probably recommend going ahead and including that update if you're comfortable doing so. While I think this PR is probably backwards compatible, it's simpler to support one implementation on the nixpkgs side.

Eventually nixpkgs will even have a hook for this deps dir building, but not until I get to refactoring mixRelease.