flake: add explicit 'systems' input

flake.nix

@@ -27,6 +27,9 @@
       owner = "numtide";
       repo = "flake-utils";
       ref = "main";
+      inputs = {
+        systems.follows = "systems";
+      };
     };
 
     home-manager = {
@@ -64,6 +67,13 @@
         nixpkgs-stable.follows = "nixpkgs";
       };
     };
+
+    systems = {
+      type = "github";
+      owner = "nix-systems";
+      repo = "default";
+      ref = "main";
+    };
   };
 
   # Can't eta-reduce a flake outputs...