ci: Run publish:versions only on final tags

The release candidate tags will end up in no versions update and failing
the job, so let's save some CI time there.

Anyway, we have an additional manual job to trigger the update on demand.

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

.gitlab-ci.yml

@@ -110,7 +110,7 @@ publish:versions:tags:
   rules:
     - if: $CI_PIPELINE_SOURCE == "schedule"
       when: never
-    - if: $CI_COMMIT_TAG
+    - if: '$CI_COMMIT_TAG =~ /^[0-9]+\.[0-9]+\.[0-9]+$/'
   extends: .publish:versions
 
 publish:versions:manual: