I just git bisected again, and the breaking change is one of these three commits:
https://github.com/NixOS/nixpkgs/commit/03a6766a6d080e064bea374f442a9f36a9a18b31
https://github.com/NixOS/nixpkgs/commit/803e87aa1e5bd071713276fd13e55854f7e5e385
https://github.com/NixOS/nixpkgs/commit/6bf1421f13d667c2997b67728cf777c6a70716a5