build: convert provided unit file to use environment for cmdline args

This should hopefully provide a more flexible approach for overriding
the service and update cmdline flags, especially considering systemd
advises against the use of `EnvironmentFile` configs and suggest using
overrides instead.

packaging/systemd/mango.service

@@ -4,7 +4,8 @@ Wants=network.target
 After=network.target
 
 [Service]
-ExecStart=/usr/bin/mango --inventory.path /opt/mango/inventory --inventory.reload-interval '5m'
+Environment="ARGS=--inventory.path /opt/mango/inventory --inventory.reload-interval '5m'"
+ExecStart=/usr/bin/mango ${ARGS}
 ExecReload=/bin/kill -SIGHUP $MAINPID
 Restart=on-failure
 KillSignal=SIGINT