Don't use a group of `nobody` for users.

This is disallowed in nixos 21.11 as it is a potential security issue.

nixos/modules/tahoe.nix

@@ -156,6 +156,10 @@ in
           nameValuePair "tahoe.introducer-${node}" {
             description = "Tahoe node user for introducer ${node}";
             isSystemUser = true;
+            group = "tahoe.introducer-${node}";
+          });
+        users.groups = flip mapAttrs' cfg.introducers (node: _:
+            nameValuePair "tahoe.introducer-${node}" {
           });
       })
       (mkIf (cfg.nodes != {}) {
@@ -287,6 +291,10 @@ in
           nameValuePair "tahoe.${node}" {
             description = "Tahoe node user for node ${node}";
             isSystemUser = true;
+            group = "tahoe.${node}";
+          });
+        users.groups = flip mapAttrs' cfg.introducers (node: _:
+            nameValuePair "tahoe.${node}" {
           });
       })
     ];