[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