[plasma-session] Don't prematurely cache systemEnvironemnt

Summary:
plasma-session goes to great lengths to make sure we have the latest env
vars from kcminit in future spawned jobs.

However one part got broke in some refactor, we capture the system
environment when we make the job, not when we're about to launch.

This means potentially kded5 and ksmserver wouldn't have the envs
kcminit set.

I've not noticed any practical impact from this.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28626