nix build . doesn't work in Ema example

[Emmanuel-PLF]

I use this example https://github.com/srid/ema/blob/master/flake.nix with GHC8107 (nixpkgs unstable and I can't build the package, with nix build . or nix run . I've got this error :
flake 'git+file:///home/manu/build/hsnix/mong-servant' does not provide attribute 'packages.x86_64-linux.default' or 'defaultPackage.x86_64-linux'. Have you got an idea ?
Thanks

[srid]

There is no default package in that one (only a default shell); you can try nix build .#ghc90 or nix build .#ghc92.