Fixed bundler installing at ruby 2.7.8

```
Installing Bundler
  Using latest Bundler for ruby-2.7.8 because the default Bundler gem is too old for that Ruby version
  Ruby 2.6-2.7 only works with Bundler 2.4
  /opt/hostedtoolcache/Ruby/2.7.8/x64/bin/gem install bundler -v ~> 2.4.0
  ERROR:  While executing gem ... (Zlib::BufError)
      buffer error
  Took   0.78 seconds
```

https://github.com/itamae-kitchen/itamae/actions/runs/7343647181/job/19997130475

.github/workflows/test_main.yml

@@ -28,7 +28,7 @@ jobs:
         with:
           ruby-version: ${{ inputs.ruby }}
           bundler-cache: true
-          bundler: default
+          bundler: none
 
       - run: bundle update