nix develop: Fix `devShells` lookup
nix develop: Fix `devShells` lookup

It currently fails with the following error:

    error: flake 'git+file://…' does not provide attribute 'devShells.x86_64-linuxhaskell', 'packages.x86_64-linux.haskell', 'legacyPackages.x86_64-linux.haskell' or 'haskell'

NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix