Fix systemd unit override file permissions

Prod-Related: PROD-35976
Change-Id: Ib6c86f0cec38f7208efbdef7511195574a957b8f
diff --git a/linux/system/service.sls b/linux/system/service.sls
index a50ae36..a5dcf8b 100644
--- a/linux/system/service.sls
+++ b/linux/system/service.sls
@@ -22,6 +22,7 @@
   - makedirs: True
   - user: root
   - group: root
+  - mode: 644
   - watch_in:
     - module: {{ service_name }}_systemd_reload
     - service: {{ service_name }}_service