cicd: pack plugins into archive

.gitlab-ci.yml

+archive:
+  image: registry.access.redhat.com/ubi8/ubi
+  script:
+    - tar --numeric-owner --owner=1001 --group=0 -czf moodle-plugins.tar.gz auth block local mod theme
+  artifacts:
+    paths:
+      - moodle-plugins.tar.xz