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/go-viper/mapstructure/v2/.envrc

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+export DIRENV_WARN_TIMEOUT=20s
+
+eval "$(devenv direnvrc)"
+
+use devenv