[ci.yaml] Explicitly use scheduler==luci (#92218)

diff --git a/.ci.yaml b/.ci.yaml
index 65fcaaf..a5a86eb 100755
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -2371,6 +2371,7 @@
         ["framework","hostonly","shard"]
       validation: verify_binaries_codesigned
       validation_name: Verify binaries codesigned
+    scheduler: luci
 
   - name: Mac web_tool_tests
     recipe: flutter/flutter_drone