fix(systemd): Make EnvironmentFile paths optional in service files #106

Job	Run time
Build charm	11m 43s
Test mode=auto	18m 55s
Test shared storage (mode=auto)	18m 43s
Test shared storage (mode=web+worker)	-1s
Test mode=web+worker	0s
Publish Charm to Edge	-1s
Cleanup artifacts	-1s
	49m 18s

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