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.yml

@@ -32,6 +32,10 @@ jobs:
           - ""
           - "--jit"
           - "--yjit"
+        include:
+          # FIXME: Workaround for bundle installing error
+          - ruby: "2.7"
+            bundler: "2.4.21"
         exclude:
           # --jit is available since MRI 2.6
           - ruby:    "2.3"
@@ -58,6 +62,7 @@ jobs:
     with:
       ruby: ${{ matrix.ruby }}
       rubyopt: ${{ matrix.rubyopt }}
+      bundler: ${{ matrix.bundler }}
     secrets:
       SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK }}
 

.github/workflows/test_main.yml

@@ -10,6 +10,10 @@ on:
         required: false
         type: string
         default: ""
+      bundler:
+        required: false
+        type: string
+        default: default
     secrets:
       SLACK_WEBHOOK:
         required: true
@@ -28,7 +32,7 @@ jobs:
         with:
           ruby-version: ${{ inputs.ruby }}
           bundler-cache: true
-          bundler: default
+          bundler: ${{ inputs.bundler != '' && inputs.bundler || 'default' }}
 
       - run: bundle update
 
@@ -65,7 +69,7 @@ jobs:
         with:
           ruby-version: ${{ inputs.ruby }}
           bundler-cache: true
-          bundler: default
+          bundler: ${{ inputs.bundler != '' && inputs.bundler || 'default' }}
 
       - run: bundle update
 
@@ -103,7 +107,7 @@ jobs:
         with:
           ruby-version: ${{ inputs.ruby }}
           bundler-cache: true
-          bundler: default
+          bundler: ${{ inputs.bundler != '' && inputs.bundler || 'default' }}
 
       - run: bundle update