ci: Use "latest" Bundler with Ruby "head"

Prevents picking up of the Gemfile's v2.4.22 which is no longer compatible with
Ruby 4.

Author: tokengeek

.github/workflows/ruby.yml

@@ -22,6 +22,9 @@ jobs:
       fail-fast: false
       matrix:
         ruby: ["2.7", "3.0", "3.1", "3.2", "3.3", "head"]
+        include:
+          - ruby: "head"
+            bundler: "latest"
     env:
       DEBUG: "true"
     steps:
@@ -32,6 +35,7 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        bundler: ${{ matrix.bundler || 'default' }}
         bundler-cache: true
     - name: Run tests
       run: bundle exec rake