add nixpkgs.nix

nixpkgs.nix

@@ -0,0 +1,27 @@
+{
+  root,
+  inputs,
+  ...
+}: {
+  nixpkgs.channels.default = {
+    # nixpkgs (stable branch)
+    pkgs = {
+      source = inputs.nixpkgs;
+      overlays =
+        [inputs.dobutterfliescry-net.overlays.default]
+        ++ import (root + "/overlays");
+      config = {
+        allowUnfree = false;
+        allowBroken = false;
+      };
+    };
+    # nixpkgs-unstable
+    upkgs = {
+      source = inputs.nixpkgs-unstable;
+      config = {
+        allowUnfree = false;
+        allowBroken = false;
+      };
+    };
+  };
+}