Hide SystemdProcessRunner behind env var
Initial use of cgroups was guarded by KDE_APPLICATIONS_AS_SCOPE
for our first rollout to perform it safely with no regressions, which worked nicely.
A massive overhaul landed which exposed some existing bugs in Plasma and the surrounding stack.
This reintroduces the previous scope-based implementation for plasma 5.19 (if systemd is available) and hides the newer behavior behind KDE_APPLICATIONS_AS_SERVICE
Edited by Henri Chain