Pass drone_dimensions as part of the main target. (#116812)

* Pass drone_dimensions as part of the main target.

Drone dimensions were passed as properties when the sharding utility is
expecting them in the target.

Bug: https://github.com/flutter/flutter/issues/116794

* Use dimensions instead of drone_dimensions.

* Use a map format.

.ci.yaml

@@ -4690,9 +4690,8 @@ targets:
       task_name: flutter_packaging
       tags: >
         ["framework", "hostonly", "shard", "mac"]
-      drone_dimensions: >
-        ["cpu=arm64"]
-
+    dimensions:
+      cpu: "arm64"
 
   - name: Windows flutter_packaging
     recipe: packaging_v2/packaging_v2