Only run publish step if called via schedule

.gitlab-ci.yml

@@ -62,3 +62,6 @@ publish:
     - pacman -Syu --needed --noconfirm qemu-headless virtualbox packer
   script:
     packer build -parallel-builds=1 -var "vagrant_cloud_token=$VAGRANT_API_TOKEN" -var 'headless=true' -var 'write_zeroes=yes' -except=vmware-iso vagrant.json
+  only:
+    variables:
+      - $SCHEDULED_PUBLISH == "TRUE"