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/source/iofs/doc.go

@@ -0,0 +1,10 @@
+/*
+Package iofs provides the Go 1.16+ io/fs#FS driver.
+
+It can accept various file systems (like embed.FS, archive/zip#Reader) implementing io/fs#FS.
+
+This driver cannot be used with Go versions 1.15 and below.
+
+Also, Opening with a URL scheme is not supported.
+*/
+package iofs