Fix ui12201 Update Manager freezes when changelog is unavailable

It was hanging after a string formatting exception caused by the formatted string being modified by the package helper. This removes the formatting expression.

The patch is a port to package helper format of user kpengboy's patch which has been applied and reported good by a number of users. Testing was therefore limited to eyeballing the changed code and checking update-manager ran OK in normal operation.

This was a Toutatis only issue.

See https://trisquel.info/en/issues/12201 And user patch https://trisquel.info/files/issues/remove-format-args.patch