Linking takes most of the time on ios release buidls

Parallelizing the ninja steps allows for linking to happen
in parallel, this  results in better utilization

Change-Id: I1311e9a5f33008bcb731c2add32e528bebfb7d8b
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/14660
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Kaushik Iska <kaushikiska@google.com>

recipes/engine.py

diff --git a/recipes/engine.py b/recipes/engine.py
index ecc8dd7..20f9d85 100644
--- a/recipes/engine.py
+++ b/recipes/engine.py
@@ -1302,8 +1302,10 @@
         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-release'