push gem only when tag is added

Author: yas-okadatech

.github/workflows/main.yml

@@ -40,6 +40,7 @@ jobs:
           ruby-version: jruby
       - name: push gem
         uses: trocco-io/push-gem-to-gpr-action@v1
+        if: startsWith( github.ref, 'refs/tags/' ) || github.event_name == 'workflow_dispatch'
         with:
           language: java
           gem-path: "./build/gems/*.gem"