Add CI/CD rule so that build is not run for tags

.gitlab-ci.yml

@@ -21,6 +21,10 @@ variables:
     - docker context create multi-arch-build
     - docker buildx create multi-arch-build --name mybuilder --bootstrap --use
   needs: []
+  rules:
+    - if: $CI_COMMIT_TAG
+      when: never
+    - when: always
 
 build-gnmi-target-ubuntu:
   script: