[dync_modules] enable iOS ddm build in the merge queue (#173094)

Similar to flutter/flutter#168233, but this time
for iOS
Follow-up to flutter/flutter#168717

As we start to work more seriously with ddm we'd like to have these
builds always available instead of on demand only.

Author: goderbauer

engine/src/flutter/.ci.yaml

@@ -484,13 +484,12 @@ targets:
 
   - name: Mac mac_ios_engine_ddm
     recipe: engine_v2/engine_v2
-    bringup: true
     timeout: 240
     enabled_branches:
       # Don't run this on release branches
       - master
-    backfill: false
     properties:
+      release_build: "true"
       add_recipes_cq: "true"
       config_name: mac_ios_engine_ddm
       $flutter/osx_sdk : >-