Merge pull request #246982 from khaneliman/nixpkgs-fmt

nixpkgs-fmt: add meta.mainProgram

pkgs/tools/nix/nixpkgs-fmt/default.nix

···
     homepage = "https://nix-community.github.io/nixpkgs-fmt";
     license = licenses.asl20;
     maintainers = with maintainers; [ zimbatm ];
+    mainProgram = "nixpkgs-fmt";
   };
 }