No need for Bundler for running ruby/spec

Author: eregon

.github/workflows/ci.yml

@@ -17,6 +17,7 @@ jobs:
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        bundler: none
     - name: Clone MSpec
       run: git clone https://github.com/ruby/mspec.git ../mspec