nix-daemon: Bump the default number of build users

While it's annoying to pollute the user database with a lot of nixbld*
users, 10 users is really too low for many modern systems.

nixos/modules/services/misc/nix-daemon.nix

···
         fi
       '';
 
-    nix.nrBuildUsers = mkDefault (lib.max 10 cfg.maxJobs);
+    nix.nrBuildUsers = mkDefault (lib.max 32 cfg.maxJobs);
 
     users.extraUsers = nixbldUsers;