Run {Platform} flutter_packaging builders on release candidates. (flutter#168762)

Closes flutter#168745.

Author: matanlurey

.ci.yaml

@@ -6945,6 +6945,7 @@ targets:
     scheduler: release
     bringup: true # https://github.com/flutter/flutter/issues/126286
     enabled_branches:
+      - flutter-\d+\.\d+-candidate\.\d+
       - beta
       - stable
     properties:
@@ -6959,6 +6960,7 @@ targets:
     timeout: 60
     scheduler: release
     enabled_branches:
+      - flutter-\d+\.\d+-candidate\.\d+
       - beta
       - stable
     properties:
@@ -6975,6 +6977,7 @@ targets:
     timeout: 60
     scheduler: release
     enabled_branches:
+      - flutter-\d+\.\d+-candidate\.\d+
       - beta
       - stable
     properties:
@@ -6991,6 +6994,7 @@ targets:
     scheduler: release
     bringup: true
     enabled_branches:
+      - flutter-\d+\.\d+-candidate\.\d+
       - beta
       - stable
     properties: