Fix downloading non-text files in EWW

* lisp/net/eww.el (eww-download-callback): Bind
coding-system-for-read to 'no-conversion', to avoid any code- or
EOL-conversions in downloaded files.  (Bug#30664)

lisp/net/eww.el

@@ -1532,7 +1532,8 @@ Differences in #targets are ignored."
                   eww-download-directory)))
       (goto-char (point-min))
       (re-search-forward "\r?\n\r?\n")
-      (write-region (point) (point-max) file)
+      (let ((coding-system-for-write 'no-conversion))
+        (write-region (point) (point-max) file))
       (message "Saved %s" file))))
 
 (defun eww-decode-url-file-name (string)