Docx writer: don't take "distArchive" from datadir.

The docx writer takes components from the distribution's
version of reference.docx when it can't find them in a
user's custom reference.docx.  (This sometimes happens
because Word will sometimes omit components needed for larger
documents when saving a simple one.)

Previously, we allowed a reference.docx in the data directory
(e.g. `~/.pandoc`) to be used as the distribution's reference.docx.
This led to a bizarre situation where pandoc would produce a
good docx using `--template ~/.pandoc/ref.docx`, but if `ref.docx`
were moved to `~/.pandoc/reference.docx`, it would then produce
a corrupted docx.

Closes #3322 (I think).

src/Text/Pandoc/Writers/Docx.hs

@@ -231,10 +231,11 @@ writeDocx opts doc@(Pandoc meta _) = do
   username <- P.lookupEnv "USERNAME"
   utctime <- P.getCurrentTime
   distArchive <- (toArchive . BL.fromStrict) <$>
-                      P.readDataFile datadir "reference.docx"
+                      P.readDataFile Nothing "reference.docx"
   refArchive <- case writerReferenceDoc opts of
                      Just f  -> toArchive <$> P.readFileLazy f
-                     Nothing -> return distArchive
+                     Nothing -> (toArchive . BL.fromStrict) <$>
+                        P.readDataFile datadir "reference.docx"
 
   parsedDoc <- parseXml refArchive distArchive "word/document.xml"
   let wname f qn = qPrefix qn == Just "w" && f (qName qn)