Add option to customize model download location

Describe the feature you'd like to request

I'm deploying nextcloud through nixOS. However, the location where the models are hard-coded to the location of the addon. That means that downloading the model will fail on NixOS, since the nix-store is read-only after deployment.

Describe the solution you'd like

Please add an option where you can configure where the models are downloaded to. IMHO, that option wouldn't be too hard to implement. Also, the flexibility to store potentially large models on a dedicated data partition will probably be useful to non-NixOS users as well.

Describe alternatives you've considered

For my usecase, I could download the models in the build-step. This would probably be the more nix-like way to deploy things. However, an option to store the model elsewhere would increase the flexibility of all deployments and follow best practices better.

Start by locating the PHP code that downloads models and the configuration entry points that determine their current location. Trace how the download path is selected and verify whether it is writable in a NixOS deployment. Done means users can configure a model directory and downloads are stored there instead of only in the add-on location.