Change name of environment variable used for new systemd launch feature

David Edmundsonrequested to merge
work/rename_systemd_guard_env into master

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))

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