Run Iceberg Spark tests only when PR title contains [iceberg] (#1976)

Author: hsiang-c

.github/workflows/iceberg_spark_test.yml

@@ -41,6 +41,7 @@ env:
 
 jobs:
   iceberg-spark-sql:
+    if: contains(github.event.pull_request.title, '[iceberg]')
     strategy:
       matrix:
         os: [ubuntu-24.04]