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/builtin_all.go

@@ -0,0 +1,13 @@
+// Copyright 2026 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"
+
+import (
+	mbits "math/bits"
+)
+
+func X__builtin_ctzll(tls *TLS, x uint64) int32 {
+	return int32(mbits.TrailingZeros64(x))
+}