`zip_get_archive_comment` returns garbled text with some zip archives if
we pass the `ZIP_FL_ENC_GUESS` flag.

Reported upstream at https://github.com/nih-at/libzip/issues/57

Using `ZIP_FL_ENC_RAW` fixes the issue and doesn't seem to break other
zip comments I have around. Since this flag doesn't seem to guarantee
UTF-8 encoding of the comment, we switch from `QString::fromUtf8()` to
`QString::fromLocal8Bit()` to play safe.