Keep host header for Grafana requests

This was breaking requests due to CSRF check.

See: https://github.com/grafana/grafana/issues/45117#issuecomment-1033842787
Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/hut/nginx.nix

@@ -53,6 +53,7 @@ in
         location /grafana {
           proxy_pass http://127.0.0.1:2342;
           proxy_redirect http:// $scheme://;
+          proxy_set_header Host $host;
           # Websockets
           proxy_http_version 1.1;
           proxy_set_header Upgrade $http_upgrade;