ci: use specific version of bundler

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/linux.yml

@@ -28,6 +28,7 @@ jobs:
       env:
         CI: true
       run: |
+        gem install bundler -v 2.4.22
         gem install rake
-        bundle install --jobs 4 --retry 3
+        bundle _2.4.22_ install --jobs 4 --retry 3
         bundle exec rake test