Can't install the correct Bundler version

I'm new to Jekyll and this GH Action, maybe I did something wrong, but if I got the docs right, then I should be able to define the right version in the workflow file and the action should use that version, no?

So I defined bundler_ver: '2.5.5' this but I still get this:

| Traceback (most recent call last):
|       2: from /usr/sbin/bundle:25:in `<main>'
|       1: from /usr/lib/ruby/2.7.0/rubygems.rb:296:in `activate_bin_path'
| /usr/lib/ruby/2.7.0/rubygems.rb:277:in `find_spec_for_exe': Could not find 'bundler' (2.5.5) required by your /home/florian/projects/jekyll/Gemfile.lock. (Gem::GemNotFoundException)
| To update to the latest version installed on your system, run `bundle update --bundler`.
| To install the missing version, run `gem install bundler:2.5.5`

Am I doing something wrong or is that a bug?