Drop use of qtpaths in startkde

Summary:
Qtpaths doesn't work on some distros (Ubuntu for example, where it's
shipped with dev tools) and it's quite a slow way to effectively just read
an environment variable.

This is a rework of d90c63ac that
also correctly includes system directories.

Test Plan:
Put shell script in ~/.config/plasma-workspace/env
put shell script /etc/xdg/plasma-workspace/env
Rebooted. Confirmed both were run

Checked earlier 'if' statement works manually in bash.

Reviewers: #plasma

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9478