Enable proxy for Grafana too

The alerts need to contact the slack endpoint, so we add the proxy
environment variables to the grafana systemd service.

Reviewed-By: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/hut/monitoring.nix

@@ -18,6 +18,9 @@
     };
   };
 
+  # Make grafana alerts also use the proxy
+  systemd.services.grafana.environment = config.networking.proxy.envVars;
+
   services.prometheus = {
     enable = true;
     port = 9001;