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-jwt/jwt/v5/token_option.go

@@ -0,0 +1,5 @@
+package jwt
+
+// TokenOption is a reserved type, which provides some forward compatibility,
+// if we ever want to introduce token creation-related options.
+type TokenOption func(*Token)