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/cpu/runtime_auxv.go

@@ -0,0 +1,16 @@
+// Copyright 2023 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 cpu
+
+// getAuxvFn is non-nil on Go 1.21+ (via runtime_auxv_go121.go init)
+// on platforms that use auxv.
+var getAuxvFn func() []uintptr
+
+func getAuxv() []uintptr {
+	if getAuxvFn == nil {
+		return nil
+	}
+	return getAuxvFn()
+}