Fix up nix flake build system

.gitignore

@@ -1,2 +1,4 @@
 DotnetSystemd/bin/**
 DotnetSystemd/obj/**
+.nuget-packages
+result

flake.nix

@@ -17,7 +17,7 @@
       ];
 
       perSystem =
-        { pkgs, ... }:
+        { system, pkgs, ... }:
         let
           dotnetRuntime = pkgs.dotnet-runtime_10;
         in
@@ -25,7 +25,7 @@
           packages.default = pkgs.buildDotnetModule rec {
             pname = "dotnet-systemd";
             version = "0.1.0";
-            src = "./DotnetSystemd";
+            src = ./DotnetSystemd;
 
             nugetDeps = ./DotnetSystemd/deps.json;