[fuchsia] Ensure flutter_apps in the same BUILD.gn have separate scratch...

[fuchsia] Ensure flutter_apps in the same BUILD.gn have separate scratch directories for building the FLX. (#3720)

build/flutter_app.gni

@@ -157,7 +157,7 @@ template("flutter_app") {
       "--flutter-tools",
       rebase_path(flutter_tools_bin),
       "--working-dir",
-      rebase_path("$target_gen_dir/build"),
+      rebase_path("$target_gen_dir/$target_name/build"),
       "--app-dir",
       rebase_path("."),
       "--packages",
@@ -322,7 +322,7 @@ template("flutter_aot_app") {
       "--flutter-tools",
       rebase_path(flutter_tools_bin),
       "--working-dir",
-      rebase_path("$target_gen_dir/build"),
+      rebase_path("$target_gen_dir/$target_name/build"),
       "--app-dir",
       rebase_path("."),
       "--packages",