Cannot save as JPEG XL (.JXL)

I am having the same issue as https://github.com/PintaProject/Pinta/issues/1857:

>**Description**
>The build of gdk-pixbuf that ships with the Flatpak version does not support saving as JXL.
>
>**To Reproduce**
>
>1. Draw anything
>2. Save it as JXL
>3. Get errored

Here is the error I receive:
```
GLib.GException: This build of gdk-pixbuf does not support saving the image format: jxl
at GdkPixbuf.Pixbuf.SaveToStreamv(OutputStream stream, String type, String[] optionKeys, String[] optionValues, Cancellable cancellable)
at Pinta.Core.GdkPixbufFormat.DoSave(Pixbuf pb, File file, String fileType, Window parent)
at Pinta.Core.GdkPixbufFormat.Export(Document document, File file, Window parent)
at Pinta.Actions.SaveDocumentImplmentationAction.SaveFile(Document document, File file, FormatDescriptor format, Window parent)
at Pinta.Actions.SaveDocumentImplmentationAction.SaveFileAs(Document document)
at Pinta.Actions.SaveDocumentImplmentationAction.Activated(FileActions sender, DocumentSaveEventArgs e)
at Pinta.Core.AsyncEventHandlerExtensions.InvokeSequential[TSender,TArgs,TResult](Returning`1 handlerBundle, TSender sender, TArgs args)
at Pinta.Core.FileActions.RaiseSaveDocument(Document document, Boolean saveAs)
at Pinta.Actions.SaveDocumentAsAction.Activated(Object sender, EventArgs e)
at System.Threading.Tasks.Task.<>c.b__124_0(Object state)
at GLib.Internal.MainLoopSynchronizationContext.<>c__DisplayClass1_0.b__0()
```

I am using version 3.1.2, running LMDE 7 (Debian 13) on the host. Let me know if you need anything else.

**Edit:** `libjxl0.11` and `libjxl-dev` are installed on the host system.

Start at Pinta.Core.GdkPixbufFormat in the Save As flow and inspect how the Flatpak build supplies gdk-pixbuf and JPEG XL support. Reproduce saving an image as JXL in the Flatpak; done means the Flatpak can save JXL without the reported gdk-pixbuf exception.

Written by the indexing model from the issue text.