flake.nix: add cargo-edit to devshell

flake.nix

@@ -31,6 +31,7 @@
           (pkgs.rust-bin.stable.latest.default.override {
             extensions = [ "rust-src" ];
           })
+          cargo-edit
           bacon
         ];
         inputsFrom = with self.packages.${system}; [ tatoeba images ];