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/pelletier/go-toml/v2/.gitignore

@@ -0,0 +1,7 @@
+test_program/test_program_bin
+fuzz/
+cmd/tomll/tomll
+cmd/tomljson/tomljson
+cmd/tomltestgen/tomltestgen
+dist
+tests/