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/modernc.org/libc/libc_musl_linux_ppc64le.go

@@ -0,0 +1,12 @@
+// Copyright 2023 The Libc Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package libc // import "modernc.org/libc"
+
+type long = int64
+
+type ulong = uint64
+
+// RawMem represents the biggest byte array the runtime can handle
+type RawMem [1<<50 - 1]byte