Use QTemporaryDir::filePath() instead of path concatenation

Summary:
This is the preferred way to get a file path in a temporary
directory. It's easy to forget to append '/' when using
QTemporaryDir::path().

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10038