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/golang.org/x/sys/plan9/pwd_plan9.go

@@ -0,0 +1,19 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package plan9
+
+import "syscall"
+
+func fixwd() {
+	syscall.Fixwd()
+}
+
+func Getwd() (wd string, err error) {
+	return syscall.Getwd()
+}
+
+func Chdir(path string) error {
+	return syscall.Chdir(path)
+}