Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/golang.org/x/sys/windows/mksyscall.go

@@ -0,0 +1,9 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build generate
+
+package windows
+
+//go:generate go run golang.org/x/sys/windows/mkwinsyscall -output zsyscall_windows.go eventlog.go service.go syscall_windows.go security_windows.go setupapi_windows.go