Change name of environment variable used for new systemd launch feature

Initial use of cgroups was guarded by an environment variable 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 patch renames the environment so we can do a slow-roll out again ensuring user experience stays regression free.

(counter patch to reverting everything as per !119 (closed))