Run windows_arm build only on main branch. (#40546)

Run windows_arm build only on main branch.

.ci.yaml

diff --git a/.ci.yaml b/.ci.yaml
index 8315470..9e5b32c 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -513,6 +513,9 @@
   - name: Windows windows_arm_host_engine
     recipe: engine_v2/engine_v2
     timeout: 60
+    enabled_branches:
+      # Don't run this on release branches
+      - main
     properties:
       release_build: "true"
       config_name: windows_arm_host_engine