nixos/system-nixconf: include best-effort {min,max}-free by default

nix/nixosModules/common/system-nixconf.nix

@@ -8,6 +8,10 @@
     package = lib.mkDefault pkgs.nix;
 
     settings = {
+      # Try to have at least 10GB of space by default
+      min-free = lib.mkDefault (5 * 1024 * 1024 * 1024);
+      max-free = lib.mkDefault (10 * 1024 * 1024 * 1024);
+
       extra-substituters = [
         "https://nix-community.cachix.org"
       ];