[systemd] Use env to set snapclient device

systemd/.config/systemd/user/snapclient.service

@@ -3,7 +3,8 @@ Description=Snapclient
 After=network.target
 
 [Service]
-ExecStart=/usr/bin/snapclient -h snapcast.service
+EnvironmentFile="/etc/snapclient.env"
+ExecStart=/usr/bin/snapclient -h snapcast.service $EXTRA_ARGS
 ExecStop=kill $(pidof snapclient)
 Restart=on-failure