morph build doesn't currently work with nixpkgs-unstable

[kolloch]

Head ups: I get build failures when using nixpkgs-unstable. Whether I build morph with the recommended nix-shell approach or with:

let morph = pkgs.callPackage "${sources.morph}/nix-packaging" {};
in morph.bin

(maybe that should be documented since it is easier than the advertised shell workflow?)

That works great with nixpkgs-20.03. It results in this error on nixpkgs-unstable:

builder for '/nix/store/xghn18r6kpw9msv47k9bgh91vyzxmvbz-morph-unstable-dev.drv' failed to produce output path '/nix/store/b45ri75n82pz78rink6vcfivyrmb40d7-morph-unstable-dev-bin'
error: build of '/nix/store/xghn18r6kpw9msv47k9bgh91vyzxmvbz-morph-unstable-dev.drv' failed

Full Logs

[kolloch]

Thank you, @srhb!