Only include the environment file if Slack alerter is enabled

nixos/modules/monitoring/server/grafana.nix

@@ -81,7 +81,11 @@ in {
     # Port 80 for ACME ssl retrieval only. 443 for nginx -> grafana.
     networking.firewall.allowedTCPPorts = [ 80 443 ];
 
-    systemd.services.grafana.serviceConfig.EnvironmentFile = cfg.grafanaEnvironmentFile;
+    # We pass the secret Slack URL using an environment variable.
+    systemd.services.grafana.serviceConfig.EnvironmentFile =
+      if cfg.enableSlackAlert
+      then [ cfg.grafanaEnvironmentFile ]
+      else [ ];
 
     services.grafana = {
       enable = true;