features/flatpak: add support for flatpak-builder

features/flatpak.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ lib, pkgs, ... }:
 {
   services.flatpak = {
     enable = true;
@@ -23,4 +23,9 @@
       }
     ];
   };
+
+  environment.systemPackages = with pkgs; [
+    flatpak-builder
+    appstream
+  ];
 }