try yet another way to suppress experimental warnings in Ruby 2

.github/workflows/ci.yml

@@ -156,9 +156,6 @@ jobs:
     - name: Set Rouge version
       if: matrix.rouge-version
       run: echo 'ROUGE_VERSION=${{ matrix.rouge-version }}' >> $GITHUB_ENV
-    - name: Suppress experimental warnings (Ruby 2.7)
-      if: matrix.ruby == '2.7'
-      run: echo 'RUBYOPT=-W:no-experimental' >> $GITHUB_ENV
     - name: Install dependencies
       run: bundle --jobs 3 --retry 3
     - name: Switch to upstream dependencies
@@ -170,4 +167,4 @@ jobs:
         bundle config --local clean true
         bundle --jobs 3 --retry 3
     - name: Run tests
-      run: bundle exec ruby -w $(bundle exec ruby -e 'print File.join Gem.bindir, %q(rake)') spec
+      run: bundle exec ruby -w -W:no-experimental $(bundle exec ruby -e 'print File.join Gem.bindir, %q(rake)') spec