fix drush installation task

[doman18]

Update required drush version from 11 to 12 (see: https://www.drupal.org/node/3403057)

Add check for drush binary file existence and replace "when" conditional in drush installation task. This works better with progressive playbook runs (when you run playbook after each change) in which composer json file already exists. Offcourse its still not perfect as it checks only file's existence, without checking it's version. Yet for learning purposes i think its a sufficient compromise between reliability and simplicity.

[doman18]

Pipeline failed but its seems that not because of my changes.

[geerlingguy]

Thanks and yeah, I need to fix up a couple of other issues there. I'll try to loop this into a few other fixes in my next pass on the book, so I can sync the manuscript to this repo.

[brendon-stephens]

Thankyou for the fix.