kopt: honor TESSDATA_PREFIX environment variable

Don't override it by default, but honor it if present.

frontend/document/koptinterface.lua

@@ -20,7 +20,10 @@ local util = require("util")
 
 local KoptInterface = {
     ocrengine = "ocrengine",
-    tessocr_data = DataStorage:getDataDir() .. "/data",
+    -- If `$TESSDATA_PREFIX` is set, don't override it: let libk2pdfopt honor it
+    -- (which includes checking for data in both `$TESSDATA_PREFIX/tessdata` and
+    -- in `$TESSDATA_PREFIX/` on more recent versions).
+    tessocr_data = not os.getenv('TESSDATA_PREFIX') and DataStorage:getDataDir().."/data" or nil,
     ocr_lang = "eng",
     ocr_type = 3, -- default 0, for more accuracy use 3
     last_context_size = nil,