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/skip2/go-qrcode/.travis.yml

@@ -0,0 +1,8 @@
+language: go
+
+go:
+ - 1.7
+
+script:
+ - go test -v ./...
+