Fix kwin.upd not running if python 2 not installed
Summary:
If the listed interpreter does not exist, kconf_update skips the update.
The update script uses python3, so declare that properly.
Test Plan: Saw the error by accident, it's gone now.
Reviewers: Plasma, zzag
Reviewed By: Plasma, zzag
Subscribers: kwin, plasma-devel
Tags: KWin
Differential Revision: https://phabricator.kde.org/D26942