Add back futures for ios_release builds

It looks like these were actually helpful.

Change-Id: Ib183444b3de4e4f56a025597238f0d88adf34249
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/28121
Reviewed-by: Dan Field <dnfield@google.com>
Commit-Queue: Zach Anderson <zra@google.com>
diff --git a/recipes/engine/engine.py b/recipes/engine/engine.py
index 5136ba9..165be30 100644
--- a/recipes/engine/engine.py
+++ b/recipes/engine/engine.py
@@ -1386,8 +1386,10 @@
         api, '--ios', '--runtime-mode', 'release', '--bitcode', '--no-goma',
         '--ios-cpu=arm'
     )
-    AutoninjaBuild(api, 'ios_release')
-    AutoninjaBuild(api, 'ios_release_arm')
+    x64_release = api.futures.spawn(AutoninjaBuild, api, 'ios_release')
+    arm64_release = api.futures.spawn(AutoninjaBuild, api, 'ios_release_arm')
+    for rel_future in api.futures.iwait([x64_release, arm64_release]):
+      rel_future.result()
     PackageIOSVariant(
         api, 'release', 'ios_release', 'ios_release_arm', 'ios_debug_sim',
         'ios_debug_sim_arm64', 'ios-release'