nixos/paperless: Always buidl tesseract with english

With the changes introduced in #303388 tesseract would only be compiled
with the languages defined in `PAPERLESS_OCR_LANGUAGE`. However, english
is always required, making tesseract fail to build when only non-english
languages are defined in tesseract:

```
eng.traineddata must be present in tessdata for Tesseract to work
```

nixos/modules/services/misc/paperless.nix

@@ -203,9 +203,12 @@ in
       apply = pkg: pkg.override {
         tesseract5 = pkg.tesseract5.override {
           # always enable detection modules
+          # tesseract fails to build when eng is not present
           enableLanguages = if cfg.settings ? PAPERLESS_OCR_LANGUAGE then
-            [ "equ" "osd" ]
+            lists.unique (
+              [ "equ" "osd" "eng" ]
               ++ lib.splitString "+" cfg.settings.PAPERLESS_OCR_LANGUAGE
+            )
           else null;
         };
       };