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/cpu_loong64.s

@@ -0,0 +1,13 @@
+// Copyright 2025 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.
+
+#include "textflag.h"
+
+// func get_cpucfg(reg uint32) uint32
+TEXT ·get_cpucfg(SB), NOSPLIT|NOFRAME, $0
+	MOVW	reg+0(FP), R5
+	// CPUCFG R5, R4 = 0x00006ca4
+	WORD	$0x00006ca4
+	MOVW	R4, ret+8(FP)
+	RET