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