Integration tests fail when CPATH is set to $CONDA_PREFIX/include

```bash
LCOMPILERS_ASSERT failed: /Users/ubaid/Desktop/OpenSource/lc/src/libasr/../libasr/asr.h
function down_cast(), line number 40 at 
f != nullptr
```

This occurs at the following https://github.com/lcompilers/lc/blob/9c855f961ac54f70199e21cb4ff0388531485d47/src/lc/clang_ast_to_asr.h#L703

In the above we have `callee` being down_casted but the callee is passed as a `nullptr`.