Run WebGL build task automatically only for tags, otherwise manually (long build time).

.gitlab-ci.yml

@@ -79,6 +79,10 @@ build-StandaloneWindows64:
 
 build-WebGL:
   <<: *build
+  rules:
+    - if: $CI_COMMIT_TAG
+      when: always
+    - when: manual
   image: unityci/editor:2020.3.4f1-webgl-0
   variables:
     BUILD_TARGET: WebGL