Make known-folders a flake input

Simpler than using `fetchFromGitHub`, we can have `nix` manage the
version instead

flake.nix

@@ -9,9 +9,12 @@
     gitignore.inputs.nixpkgs.follows = "nixpkgs";
 
     flake-utils.url = "github:numtide/flake-utils";
+
+    known-folders.url = "github:ziglibs/known-folders";
+    known-folders.flake = false;
   };
   
-  outputs = {self, nixpkgs, zig-overlay, gitignore, flake-utils }:
+  outputs = {self, nixpkgs, zig-overlay, gitignore, flake-utils, known-folders }:
     let
       systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
       inherit (gitignore.lib) gitignoreSource;
@@ -19,21 +22,6 @@
       let
         pkgs = nixpkgs.legacyPackages.${system};
         zig = zig-overlay.packages.${system}.master.latest;
-        known-folders = pkgs.stdenvNoCC.mkDerivation {
-          name = "known-folders";
-          src = pkgs.fetchFromGitHub {
-            owner = "ziglibs";
-            repo = "known-folders";
-            rev = "9db1b99219c767d5e24994b1525273fe4031e464";
-            sha256 = "sha256-eqaZxIax8C75L2UwDbVKSUZ7iThm/iWblfoaTfPyHLM=";
-          };
-          dontConfigure = true;
-          dontInstall = true;
-          buildPhase = ''
-            mkdir -p $out
-            cp known-folders.zig $out
-          '';
-        };
       in rec {
         packages.default = packages.zls;
         packages.zls = pkgs.stdenvNoCC.mkDerivation {
@@ -41,7 +29,6 @@
           version = "master";
           src = gitignoreSource ./.;
           nativeBuildInputs = [ zig ];
-          buildInputs = [ known-folders ];
           dontConfigure = true;
           dontInstall = true;
           buildPhase = ''