chore: Install rspec gem for running tests in CI workflow

Author: hahwul

.github/workflows/ci.yml

@@ -24,5 +24,7 @@ jobs:
       with:
         ruby-version: ${{ matrix.ruby-version }}
         bundler-cache: false # runs 'bundle install' and caches installed gems automatically
+    - name: Install dependencies
+      run: gem install rspec
     - name: Run tests
-      run: bundle exec rspec
+      run: rspec