create environment for flux-core t2410-sdexec-memlimit.t to run

[garlick]

Problem: t2410-sdexec-memlimit.t is skipped in flux-core's regular CI.

As discussed in flux-framework/flux-core#5972, it requires a systemd user instance to be running and also delegation of the cgroup memory controller, otherwise it is automatically skipped. If we can somehow provide the correct environment, the test will run.

[wihobbs]

I don't know if we're going to be able to do this because the service user doesn't have root access, so it can't run systemctl start user@60943.service. We have rootful gitlab runners in LC, but they're pretty niche and usually for deployment workflows.

Checking the status of my user slice yields:

(s=101,d=0)  fluke108 ~ $ systemctl status user@60943.service
● user@60943.service - User Manager for UID 60943
   Loaded: loaded (/usr/lib/systemd/system/user@.service; static; vendor preset: disabled)
   Active: inactive (dead)