feat: add flake-friendly nix-index overlay

See: https://github.com/nix-community/nix-index/pull/227

overlays/default.nix

@@ -1,5 +1,34 @@
-[
+{
-  (import ../pkgs)
+  self,
-  (import ./manix.nix)
+  lib,
-  (import ./neovim-plugins.nix)
+  inputs,
-]
+  ...
+}: {
+  flake = {
+    nixosModules = rec {
+      overlays = ({ ... }: {
+        nixpkgs.overlays = [
+
+          (final: prev:
+          let
+            unstable = import inputs.unstable {
+              system = prev.system;
+              #config.allowUnfreePredicate = pkg: builtins.elem (prev.lib.getName pkg) [
+              #  "1password-cli"
+              #  "1password"
+              #  "slack"
+              #];
+            };
+          in
+          {
+            #slack = unstable.slack;
+            #vimPlugins = prev.vimPlugins // {inherit (unstable.vimPlugins) nvim-lspconfig;};
+          })
+          (import ../pkgs)
+          (import ./neovim-plugins.nix)
+          (import ./nix-index.nix)
+        ];
+      });
+    };
+  };
+}