grafana service: unbreak

Accidentally broken by 4fede53c
("nixos manuals: bring back package references").

Without this fix, grafana won't start:

$ systemctl status grafana
...
systemd[1]: Starting Grafana Service Daemon...
systemd[1]: Started Grafana Service Daemon.
grafana[666]: 2016/03/06 19:57:32 [log.go:75 Fatal()] [E] Failed to detect generated css or javascript files in static root (%!s(MISSING)), have you executed default grunt task?
systemd[1]: grafana.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: grafana.service: Unit entered failed state.
systemd[1]: grafana.service: Failed with result 'exit-code'.