Correctly set default value in systemd unit

deploy/funkwhale-worker.service

@@ -7,8 +7,9 @@ PartOf=funkwhale.target
 User=funkwhale
 # adapt this depending on the path of your funkwhale installation
 WorkingDirectory=/srv/funkwhale/api
+Environment="CELERYD_CONCURRENCY=0"
 EnvironmentFile=/srv/funkwhale/config/.env
-ExecStart=/srv/funkwhale/virtualenv/bin/celery -A funkwhale_api.taskapp worker -l INFO --concurrency=${CELERYD_CONCURRENCY-0}
+ExecStart=/srv/funkwhale/virtualenv/bin/celery -A funkwhale_api.taskapp worker -l INFO --concurrency=${CELERYD_CONCURRENCY}
 
 
 [Install]