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/unix/gccgo_c.c

@@ -0,0 +1,44 @@
+// 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.
+
+//go:build gccgo && !aix && !hurd
+
+#include <errno.h>
+#include <stdint.h>
+#include <unistd.h>
+
+#define _STRINGIFY2_(x) #x
+#define _STRINGIFY_(x) _STRINGIFY2_(x)
+#define GOSYM_PREFIX _STRINGIFY_(__USER_LABEL_PREFIX__)
+
+// Call syscall from C code because the gccgo support for calling from
+// Go to C does not support varargs functions.
+
+struct ret {
+	uintptr_t r;
+	uintptr_t err;
+};
+
+struct ret gccgoRealSyscall(uintptr_t trap, uintptr_t a1, uintptr_t a2, uintptr_t a3, uintptr_t a4, uintptr_t a5, uintptr_t a6, uintptr_t a7, uintptr_t a8, uintptr_t a9)
+  __asm__(GOSYM_PREFIX GOPKGPATH ".realSyscall");
+
+struct ret
+gccgoRealSyscall(uintptr_t trap, uintptr_t a1, uintptr_t a2, uintptr_t a3, uintptr_t a4, uintptr_t a5, uintptr_t a6, uintptr_t a7, uintptr_t a8, uintptr_t a9)
+{
+	struct ret r;
+
+	errno = 0;
+	r.r = syscall(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9);
+	r.err = errno;
+	return r;
+}
+
+uintptr_t gccgoRealSyscallNoError(uintptr_t trap, uintptr_t a1, uintptr_t a2, uintptr_t a3, uintptr_t a4, uintptr_t a5, uintptr_t a6, uintptr_t a7, uintptr_t a8, uintptr_t a9)
+  __asm__(GOSYM_PREFIX GOPKGPATH ".realSyscallNoError");
+
+uintptr_t
+gccgoRealSyscallNoError(uintptr_t trap, uintptr_t a1, uintptr_t a2, uintptr_t a3, uintptr_t a4, uintptr_t a5, uintptr_t a6, uintptr_t a7, uintptr_t a8, uintptr_t a9)
+{
+	return syscall(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9);
+}