feat: add host 'nixos-unstable' as a testing VM

flake.nix

@@ -14,5 +14,10 @@
       ];
       specialArgs = { inherit inputs; };
     };
+    nixosConfigurations.nixos-unstable = nixpkgs-unstable.lib.nixosSystem {
+      modules = [
+        ./hosts/nixos-unstable
+      ];
+    };
   };
 }