nix: add garnix cache to nixConfig

Signed-off-by: seth <getchoo@tuta.io>

flake.nix

@@ -1,6 +1,11 @@
 {
   description = "A custom launcher for Minecraft that allows you to easily manage multiple installations of Minecraft at once (Fork of MultiMC)";
 
+  nixConfig = {
+    extra-substituters = ["https://cache.garnix.io"];
+    extra-trusted-public-keys = ["cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g="];
+  };
+
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-parts = {

nix/README.md

@@ -9,7 +9,8 @@ See [Package variants](#package-variants) for a list of available packages.
 ## Installing a development release (flake)
 
 We use [garnix](https://garnix.io/) to build and cache our development builds.
-If you want to avoid rebuilds you may add the garnix cache to your substitutors.
+If you want to avoid rebuilds you may add the garnix cache to your substitutors, or use `--accept-flake-config`
+to temporarily enable it when using `nix` commands.
 
 Example (NixOS):