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/golang-migrate/migrate/v4/.dockerignore

@@ -0,0 +1,12 @@
+# Project
+FAQ.md
+README.md
+LICENSE
+.gitignore
+.travis.yml
+CONTRIBUTING.md
+MIGRATIONS.md
+docker-deploy.sh
+
+# Golang
+testing