* fix(nix): package local speech worker
* fix(nix): reuse sherpa package helper
* fix(nix): patch sherpa-onnx prebuilt binaries for NixOS libstdc++
The prebuilt sherpa-onnx-linux-x64 .so files link against libstdc++.so.6
which is not on the NixOS library path. Add autoPatchelfHook to fix ELF
RPATHs and stdenv.cc.cc.lib to provide the C++ runtime, resolving the
"Failed to load model because protobuf parsing failed" SIGABRT at
speech worker startup.