Port the SqlPodcastChannel query builder from QString::arg() to QTextStream.

Among the query payload is a local filesystem URL, which may
contain percent-encoded non-ASCII data, e.g. a directory name
written in Korean. This causes havoc in combiation with the
percent sign-based placeholders QString::arg() operates on.

The SqlPodcastEpisode query builder was already using
QTextStream.

BUG:333946