Add setup-ruby step for tool scripts

Author: hsbt

.github/workflows/bundled_gems.yml

@@ -38,6 +38,10 @@ jobs:
         with:
           token: ${{ (github.repository == 'ruby/ruby' && !startsWith(github.event_name, 'pull')) && secrets.MATZBOT_AUTO_UPDATE_TOKEN || secrets.GITHUB_TOKEN }}
 
+      - uses: ruby/setup-ruby@90be1154f987f4dc0fe0dd0feedac9e473aa4ba8 # v1.286.0
+        with:
+          ruby-version: 4.0
+
       - uses: ./.github/actions/setup/directories
         with:
           # Skip overwriting MATZBOT_AUTO_UPDATE_TOKEN