Trigger release build instead of running it in finalize_release

Two reasons:

1. In it's current status if fails because of `tumblr-metal` constraints.
   See https://buildkite.com/automattic/simplenote-android/builds/433#0192c195-15a6-49dd-be04-e14b04eea312
2. There is a difference from simply running the lane that builds the
   release build and running the full CI pipeline. The latter is what we
   need.

Author: mokagio

fastlane/lanes/release.rb

@@ -192,7 +192,7 @@
 
     push_to_git_remote(tags: false)
 
-    build_and_upload_release(create_release: true)
+    trigger_release_build(branch_to_build: release_branch_name)
 
     create_backmerge_prs!