feat(flake.nix/inputs): Add home-manager-unstable

- Its `nixpkgs` follows our `nixpkgs-unstable`

Author: reo101

flake.lock

@@ -25,6 +25,11 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    home-manager-unstable = {
+      url = "github:nix-community/home-manager";
+      inputs.nixpkgs.follows = "nixpkgs-unstable";
+    };
+
     nix-darwin = {
       url = "github:LnL7/nix-darwin";
       inputs.nixpkgs.follows = "nixpkgs";