Finally switch to flakes.

overlays/default.nix

@@ -0,0 +1,22 @@
+# This file defines overlays
+{inputs, ...}: {
+  # This one brings our custom packages from the 'pkgs' directory
+  additions = final: _prev: import ../pkgs {pkgs = final;};
+
+  # This one contains whatever you want to overlay
+  # You can change versions, add patches, set compilation flags, anything really.
+  # https://nixos.wiki/wiki/Overlays
+  modifications = (self: super: {
+    mpv = super.mpv.override {
+      scripts = with self.mpvScripts; [mpris sponsorblock thumbnail];
+    };
+  });
+
+  # Unstable nixpkgs and nur through 'pkgs.unstable' and 'pkgs.nur'
+  unstable-packages = final: _prev: {
+    unstable = import inputs.nixpkgs-unstable {
+      system = final.system;
+      config.allowUnfree = true;
+    };
+  };
+}