Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

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

vendor/modernc.org/libc/nofsync.go

@@ -0,0 +1,10 @@
+// Copyright 2020 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.
+
+//go:build libc.nofsync
+// +build libc.nofsync
+
+package libc // import "modernc.org/libc"
+
+const noFsync = true