Add filter for unstable branch only

Signed-off-by: Hanxi Zhang <hanxizh@amazon.com>

Author: hanxizh9910

.github/workflows/test-failure-detector.yml

@@ -3,6 +3,8 @@ name: Test Failure Detector
 on:
   workflow_run:
     workflows: ["Daily"]
+    branches:
+      - unstable
     types:
       - completed