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/golang.org/x/sys/unix/vgetrandom_linux.go

@@ -0,0 +1,13 @@
+// Copyright 2024 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 linux && go1.24
+
+package unix
+
+import _ "unsafe"
+
+//go:linkname vgetrandom runtime.vgetrandom
+//go:noescape
+func vgetrandom(p []byte, flags uint32) (ret int, supported bool)