feat: update Nix flake file template

snippets/nix-mode/__flake.nix

@@ -8,10 +8,14 @@
 
   inputs = {
     ${2:nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
-    }${3:flake-utils.url = "github:numtide/flake-utils";
-    }$4
+    systems.url = "github:nix-systems/default";}
   };
 
-  outputs = { $5 }:
-    $0
+  outputs = { self, ${3:nixpkgs, systems, }... }:
+    ${3:$(when (and (string-match-p "nixpkgs[[:space:]]*," yas-text)
+                    (string-match-p "systems[[:space:]]*," yas-text)) "let
+      eachSystem = nixpkgs.lib.genAttrs (import systems);
+    in ")}{
+      $0
+    };
 }