nix: add cc to devShell LD_LIBRARY_PATH

nix: add cc to devShell LD_LIBRARY_PATH

this fixes the error I encountered when trying to run the convert.py
script in a venv:

```
$ nix develop

[...]$ source .venv/bin/activate
(.venv)
[...]$ pip3 install -r requirements.txt
<... clipped ...>
[...]$ python3 ./convert.py
Traceback (most recent call last):
  File "/home/mhueschen/projects-reference/llama.cpp/./convert.py", line 40, in <module>
    from sentencepiece import SentencePieceProcessor
  File "/home/mhueschen/projects-reference/llama.cpp/.venv/lib/python3.11/site-packages/sentencepiece/__init__.py", line 13, in <module>
    from . import _sentencepiece
ImportError: libstdc++.so.6: cannot open shared object file: No such file or directory
```

however, I am not sure this is the cleanest way to address this linker
issue...