systemd: change u-a.service type from exec to simple #2771

Job	Run time
Check secrets / Check secret	1s
Packaging (xenial)	4m 25s
Packaging (bionic)	5m 4s
Packaging (focal)	4m 45s
Packaging (jammy)	5m 52s
Packaging (lunar)	5m 16s
Packaging (mantic)	4m 51s
Integration (bionic, lxd, ubuntu-22.04)	1h 7m 24s
Integration (focal, lxd, ubuntu-22.04)	1h 34m 56s
Integration (jammy, lxd, ubuntu-22.04)	54m 21s
Integration (lunar, lxd, ubuntu-22.04)	34m 21s
Integration (xenial, lxd, ubuntu-20.04)	1h 29m 57s
Integration (bionic, awspro, ubuntu-22.04)	41m 17s
Integration (bionic, gcppro, ubuntu-22.04)	40m 30s
Integration (bionic, awspro-fips, ubuntu-22.04)	15m 12s
	7h 48m 12s

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