Grpc.Tools' bundled linux_arm64 protoc segfaults (exit 139) under dotnet on
native arm64 runners (grpc/grpc#26032, #38538), which broke the multi-arch image
build the moment a branch carrying AgentProtocol (.proto) got tagged. Install the
distro protobuf-compiler and point PROTOBUF_PROTOC at it, arm64-only, in both the
main and agent images. amd64 keeps the bundled protoc, so its build is unchanged.