chore: add installation step for Ruby gems in CI workflow

Author: Sec-ant

.github/workflows/test.yml

@@ -23,5 +23,9 @@ jobs:
           ruby-version: "3.4" # Not needed with a .ruby-version, .tool-versions or mise.toml
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
 
+      - name: Install Ruby gems for plugin-ruby
+        run: |
+          gem install syntax_tree --no-document
+
       - name: Test
         run: pnpm -s test:ci