Use the tail of the sorted list

Change-Id: Ieadf8ad1c5a111c6e8da83285122d85e13dd18a7
diff --git a/cicd/pipelines/tasks/task_update_unstable.jenkins b/cicd/pipelines/tasks/task_update_unstable.jenkins
index e6c5134..4ab1d37 100644
--- a/cicd/pipelines/tasks/task_update_unstable.jenkins
+++ b/cicd/pipelines/tasks/task_update_unstable.jenkins
@@ -4,7 +4,7 @@
     def script = """
         aptly snapshot list --sort=time --raw \
             | grep -E '^${repository_stem}-' \
-            | head -n1
+            | tail -n1
     """
 
     return sh(returnStdout: true, script: script).trim()