flatpak: set meta.mainProgram (#358995)

Author: Mindavi

pkgs/by-name/fl/flatpak/package.nix

@@ -265,6 +265,7 @@ stdenv.mkDerivation (finalAttrs: {
     changelog = "https://github.com/flatpak/flatpak/releases/tag/${finalAttrs.version}";
     license = lib.licenses.lgpl21Plus;
     maintainers = with lib.maintainers; [ getchoo ];
+    mainProgram = "flatpak";
     platforms = lib.platforms.linux;
   };
 })