flake: add nix-community cachix as substituters (#1796)

Link: https://github.com/nix-community/stylix/pull/1796

Reviewed-by: Matt Sturgeon <matt@sturgeon.me.uk>
Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>
(cherry picked from commit 844294a58e)

flake.nix

@@ -96,6 +96,13 @@
     # keep-sorted end
   };
 
+  nixConfig = {
+    extra-substituters = [ "https://nix-community.cachix.org" ];
+    extra-trusted-public-keys = [
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+    ];
+  };
+
   outputs =
     { flake-parts, systems, ... }@inputs:
     flake-parts.lib.mkFlake { inherit inputs; } {