GitHub actions Ubuntu 24.04 prep

[fbacall]

Summary of changes

• Add GitHub actions workflow step that installs system dependencies (only imagemagick is needed for now).

Motivation and context

ubuntu-latest will point to an Ubuntu 24.04 image soon, which does not include imagemagick by default and causes some tests to fail:

 Error:
MaterialsControllerTest#test_should_get_edit_page_for_approved_editor:
Paperclip::Errors::CommandNotFoundError: Could not run the `identify` command. Please install ImageMagick.
    lib/has_image.rb:42:in `block in resolve_image_url'
    lib/has_image.rb:41:in `resolve_image_url'
    app/models/content_provider.rb:113:in `add_editor'
    test/controllers/materials_controller_test.rb:198:in `block in <class:MaterialsControllerTest>'

https://github.com/ElixirTeSS/TeSS/actions/runs/12585002718/job/35075837030

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.