CI: set bundler version in build.yaml

Got error:
The last version of bundler (>= 0) to support your Ruby & RubyGems was 2.3.26.
Try installing it with `gem install bundler -v 2.3.26`
https://github.com/manubot/manubot.org/actions/runs/3810306504/jobs/6482307652#step:4:7

.github/workflows/build.yaml

@@ -17,7 +17,7 @@ jobs:
           ruby-version: '2.5'
       - name: Install dependencies
         run: |
-          gem install bundler
+          gem install bundler --version=2.3.26
           bundle install --jobs=3
       - name: Build website
         run: bundle exec jekyll build