Add unstable overlay

overlays/unstable/default.nix

@@ -0,0 +1,17 @@
+# Snowfall Lib provides access to your current Nix channels and inputs.
+#
+# Channels are named after NixPkgs instances in your flake inputs. For example,
+# with the input `nixpkgs` there will be a channel available at `channels.nixpkgs`.
+# These channels are system-specific instances of NixPkgs that can be used to quickly
+# pull packages into your overlay.
+#
+# All other arguments for this function are your flake inputs.
+{ nixpkgs-unstable, ... }:
+final: prev: {
+  # unstable = nixpkgs-unstable.legacyPackages.${prev.system};
+  # use this variant if unfree packages are needed:
+  unstable = import nixpkgs-unstable {
+    system = prev.system;
+    config.allowUnfree = true;
+  };
+}