Fix space usage dashboard: Use the same colors as for other machines

bind-mounting the nix store r/o made that FS turn up twice in the list,
resulting in the graph showing the wrong colors for the machines.

nixos/modules/monitoring/server/grafana-dashboards/resources-overview.json

@@ -1149,7 +1149,7 @@
       "pluginVersion": "8.4.5",
       "targets": [
         {
-          "expr": "1 - (node_filesystem_avail_bytes / node_filesystem_size_bytes) > 0.1",
+          "expr": "1 - (node_filesystem_avail_bytes{mountpoint!='/nix/store'} / node_filesystem_size_bytes{mountpoint!='/nix/store'}) > 0.1",
           "format": "time_series",
           "hide": false,
           "instant": false,