[about-distro] run variant through os-release
Summary:
this allows getting the pretty variant string from os-release if it was
not overridden via kcm-about-distrorc
CHANGELOG: About System now displays "variant" information from os-release
Test Plan: variant can be set through os-release
Reviewers: apol
Reviewed By: apol
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D19182