I'm not sure if this helps solve your issue, but I had problems getting Imagick to work in PHP and Laravel and ended up on this thread. I managed to solve it by:

• Adding "ext-imagick": "*" to composer.json, to let Nixpack know to activate the extension

• The package I was using (spatie/pdf-to-image) also requires Ghostscript. For that I needed to add it as dependency for the Nixpack build:
  create a `nixpacks.toml` file, and add the following:

[phases.setup]
nixPkgs = ['...', 'ghostscript']

Hopefully this ends up being helpful to someone!