Only run security-scanner on scheduled builds

.gitlab-ci.yml

@@ -51,6 +51,8 @@ security_scanner:
     - set -x
     - export GITLAB_URL="$(echo "$CI_PROJECT_URL" | grep -Eo '^https?://[^/]*')"
     - security-scanner stretch
+  only:
+    - schedules
 
 .squashfs_template: &squashfs_template
   stage: build