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/google/go-tpm/legacy/tpm2/open_windows.go

@@ -0,0 +1,39 @@
+//go:build windows
+
+// Copyright (c) 2018, Google LLC All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tpm2
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/google/go-tpm/tpmutil"
+	"github.com/google/go-tpm/tpmutil/tbs"
+)
+
+// OpenTPM opens a channel to the TPM.
+func OpenTPM() (io.ReadWriteCloser, error) {
+	info, err := tbs.GetDeviceInfo()
+	if err != nil {
+		return nil, err
+	}
+
+	if info.TPMVersion != tbs.TPMVersion20 {
+		return nil, fmt.Errorf("openTPM: device is not a TPM 2.0")
+	}
+
+	return tpmutil.OpenTPM()
+}