Try to fix chapter 10 deployment example (#429)

[FantasyTeddy]

I tried to fix the following issues:

• Using ubuntu-20.04 instead of ubuntu-latest (which defaults to Ubuntu 22.04 since November last year) in the CI script.
  This temporarily fixes the Failed to connect to bus: No such file or directory problem, that the scheduled CI builds are facing too. It seems to be related to the Docker cgroups change you also blogged about. However I'm not smart enough to actually fix the underlying issue. => Fixed by a4b7cfb
• Try to fix the chapter 10 "Deployment" example with these changes:
  • Use the older Ruby version 2.6.0 to fix the NoMethodError: undefined method 'new' for BigDecimal:Class error.
  • Install the newer bundler version 2.1.4 that is required by the application.
  • Set the Git config setting safe.directory to '*' and clone the application without become: true to prevent some weird user related errors. (There is probably also a more correct version to this, but it works.)
  • Do not create the app_directory during provisioning (not strictly necessary).
• Enable the deployments.yml test again in the CI configuration.
• Set fail-fast to false, mainly because the drupal.yml test is currently failing and I still want to see if everything else works.
  The alternative would be to temporarily disable the drupal.yml test in the CI config.

[geerlingguy]

If you'd like to re-test again, I would take a closer look at this PR—would love to get that example up and running again (but I do want to make sure I run it on ubuntu-latest, as locking in a version means I'll be blindsided whenever the version of Ubuntu I'm locked into goes away.

[FantasyTeddy]

I removed my first commit that changed from ubuntu-latest to ubuntu-20.04. It all seems to work just fine.