Improve rdoc buildin step

Author: st0012

.github/workflows/ruby-core.yml

@@ -52,8 +52,9 @@ jobs:
           path: ruby/rdoc
       - name: Install RDoc locally
         run: |
-          gem build rdoc
-          gem install *.gem --install-dir ../ruby/.bundle
+          bundle install
+          bundle exec rake build
+          gem install pkg/rdoc-*.gem --install-dir ../ruby/.bundle
         working-directory: ruby/rdoc
       - name: Generate Documentation with RDoc
         run: make html V=1