Fix alchemy:generate:thumbnails task

[afdev82]

What is this pull request for?

If I try to run the alchemy:generate:thumbnails task, I get the following error:

rake aborted!
Don't know how to build task 'alchemy_dragonfly_s3:generate:picture_thumbnails' (See the list of available tasks with `rake --tasks`)
Did you mean?  alchemy:generate:picture_thumbnails

Tasks: TOP => alchemy:generate:thumbnails
(See full trace by running task with --trace)

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[tvdeyen]

Thank you 🙏🏻

Test failures are not related.

[tvdeyen]

Rebased with latest main to fix CI issues

[tvdeyen]

Thanks @afdev82