Commit d11b181a broke dwarffs builds on flakes

[bqv]

At least in my flake, using d11b181 or later results in this:

builder for '/nix/store/ycdc9w8z0c7zyr0w0rl50g99bcv0gyvf-dwarffs-0.1.20200422.drv' failed with exit code 2; last 10 log lines:
  configuring
  no configure script, doing nothing
  building
  build flags: SHELL=/nix/store/z1l2n01xdfcm9bkkir83c32mkpvv51zq-bash-4.4-p23/bin/bash
  g++ -c -g -std=c++17 -Wall -Os dwarffs.cc -o dwarffs.o
  dwarffs.cc:7:10: fatal error: filetransfer.hh: No such file or directory
      7 | #include "filetransfer.hh"
        |          ^~~~~~~~~~~~~~~~~
  compilation terminated.
  make: *** [Makefile:5: dwarffs.o] Error 1
cannot build derivation '/nix/store/yydmc8b7xh90f7ys8yh4ka5xq9l8axpn-dwarffs-0.1.20200422_fish-completions.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/j87w2q57qjz3qn7bhqj25kx0637706v7-stage-2-init.sh.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/82a7s0a7xrfq8syh4hfdl3qcd58k7ls9-system-generators.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/myv33pqyhawrkn1iwkfdb3qbvb1mbynd-system-path.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/wxwqvh2qs9c2066dmngavnva15w1gw6b-system-shutdown.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/bky0zbvmx4mjd6gs4x3l1d23nscc7xw5-system-units.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/j8ymz5kx22vnvrx6a66mvjd6n6h2jq19-user-units.drv': 1 dependencies couldn't be built

My fault or yours? :p

[edolstra]

Are you using the overlay? In that case you also need to use the overlay from the nix flake since dwarffs only builds against current nix master.

[bqv]

  inputs.nix.url = "github:nixos/nix/flakes";
  inputs.dwarffs.url = "github:edolstra/dwarffs";
  inputs.dwarffs.inputs.nix.follows = "nix";

Presumably this should work? I've tested with this and it throws that error still.

FYI I'm using the nixosModule (which indirectly uses the overlay)