Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/google.golang.org/protobuf/internal/flags/flags.go

@@ -0,0 +1,24 @@
+// Copyright 2018 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 flags provides a set of flags controlled by build tags.
+package flags
+
+// ProtoLegacy specifies whether to enable support for legacy functionality
+// such as MessageSets, and various other obscure behavior
+// that is necessary to maintain backwards compatibility with proto1 or
+// the pre-release variants of proto2 and proto3.
+//
+// This is disabled by default unless built with the "protolegacy" tag.
+//
+// WARNING: The compatibility agreement covers nothing provided by this flag.
+// As such, functionality may suddenly be removed or changed at our discretion.
+const ProtoLegacy = protoLegacy
+
+// LazyUnmarshalExtensions specifies whether to lazily unmarshal extensions.
+//
+// Lazy extension unmarshaling validates the contents of message-valued
+// extension fields at unmarshal time, but defers creating the message
+// structure until the extension is first accessed.
+const LazyUnmarshalExtensions = ProtoLegacy