Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

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

vendor/github.com/fxamacker/cbor/v2/omitzero_go124.go

@@ -0,0 +1,8 @@
+// Copyright (c) Faye Amacker. All rights reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+//go:build go1.24
+
+package cbor
+
+var jsonStdlibSupportsOmitzero = true