Use the bundled bundler

Author: nobu

.github/workflows/ubuntu.yml

@@ -251,6 +251,8 @@ jobs:
       - name: Run ruby-bench
         run: ruby run_benchmarks.rb -e "ruby::../build/install/bin/ruby" ${{ matrix.bench_opts }}
         working-directory: ruby-bench
+        env:
+          BUNDLER_VERSION: 1
 
       - uses: ./.github/actions/slack
         with: