flutter.buildFlutterApplication: Use extraPackageConfigSetup

pkgs/build-support/flutter/default.nix

···
     '';
   };
 
+  extraPackageConfigSetup = ''
+    # https://github.com/flutter/flutter/blob/3.13.8/packages/flutter_tools/lib/src/dart/pub.dart#L755
+    if [ "$(yq '.flutter.generate // false' pubspec.yaml)" = "true" ]; then
+      jq '.packages |= . + [{
+        name: "flutter_gen",
+        rootUri: "flutter_gen",
+        languageVersion: "2.12",
+      }]' "$out" | '${moreutils}/bin/sponge' "$out"
+    fi
+  '';
+
   nativeBuildInputs = (args.nativeBuildInputs or [ ]) ++ [ wrapGAppsHook jq yq moreutils ];
   buildInputs = (args.buildInputs or [ ]) ++ [ glib ];
 
···
     runHook preBuild
 
     mkdir -p build/flutter_assets/fonts
-
-    # https://github.com/flutter/flutter/blob/3.13.8/packages/flutter_tools/lib/src/dart/pub.dart#L755
-    if [ "$(yq '.flutter.generate // false' pubspec.yaml)" = "true" ]; then
-      jq '.packages |= . + [{
-        name: "flutter_gen",
-        rootUri: "flutter_gen",
-        languageVersion: "2.12",
-      }]' .dart_tool/package_config.json | sponge .dart_tool/package_config.json
-    fi
 
     flutter build linux -v --release --split-debug-info="$debug" ${builtins.concatStringsSep " " (map (flag: "\"${flag}\"") flutterBuildFlags)}