Pass dimension explicitly to mac x64 packaging. (#117172)

Without passing this dimension there is a chance that the build will
run on a arm64 machine.

Author: godofredoc

.ci.yaml

@@ -4750,6 +4750,8 @@ targets:
       task_name: flutter_packaging
       tags: >
         ["framework", "hostonly", "shard", "mac"]
+    dimensions:
+      cpu: "x86"
 
 
   - name: Mac_arm64 flutter_packaging