chore(nixbuild): uncomment substituters and add buffer size

- Uncommented the `substituters` configuration to enable builder access settings in `nixbuild.nix`.
- Added `download-buffer-size` setting to optimize Nix download performance.

systems/nixbuild.nix

@@ -30,14 +30,12 @@
   ];
 
   nix.settings = {
-
     substituters = [
       "https://cache.nixos.org"
       "ssh-ng://harald@m4?ssh-key=/etc/ssh/nix-builder-key"
       "ssh-ng://harald@rialo?ssh-key=/etc/ssh/nix-builder-key"
       "ssh-ng://harald@sgx?ssh-key=/etc/ssh/nix-builder-key"
     ];
-
     builders-use-substitutes = true;
   };
 
@@ -46,4 +44,6 @@
     set -gx PATH /nix/var/nix/profiles/default/bin $PATH
     set -gx PATH /run/current-system/sw/bin $PATH
   '';
+
+  nix.settings.download-buffer-size = 524288000;
 }