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/devenv.nix

@@ -0,0 +1,14 @@
+{
+  pkgs,
+  ...
+}:
+
+{
+  languages = {
+    go.enable = true;
+  };
+
+  packages = with pkgs; [
+    golangci-lint
+  ];
+}