nsm: Switch periodic jobs from timed to pollscm

So we don't unnecessarily build images if there is no change in Git.

Change-Id: I5e513a61cf382840cd68dd6e805b007acb728e44
diff --git a/jjb/nsm/nordix-nsm-meridio-periodic.yaml b/jjb/nsm/nordix-nsm-meridio-periodic.yaml
index 556a15e..858e7dd 100644
--- a/jjb/nsm/nordix-nsm-meridio-periodic.yaml
+++ b/jjb/nsm/nordix-nsm-meridio-periodic.yaml
@@ -85,7 +85,8 @@
           wipe-workspace: true
 
     triggers:
-      - timed: '@midnight'
+      - pollscm:
+          cron: '@midnight'
 
     wrappers:
       - build-timeout: