[INFRA]: Subdirectoy nvbench_helper is added twice

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### Overview

Subdirectoy nvbench_helper is added twice when using `--preset=cub-tune -DCCCL_ENABLE_NVBENCH_HELPER=ON`

### Details

Running `cmake .. --preset=cub-tune -DCCCL_ENABLE_NVBENCH_HELPER=ON` from a build folder leads to the following issue at the end of the cmake configuration:

```sh
CMake Error at CMakeLists.txt:159 (add_subdirectory):
The binary directory

/home/mschoos/Code/cuda/cccl-tests/cccl/build6/nvbench_helper

is already used to build a source directory. It cannot be used to build
source directory

/home/mschoos/Code/cuda/cccl-tests/cccl/nvbench_helper

Specify a unique binary directory name.

-- Configuring incomplete, errors occurred!
```

The subdirectory nvbench_helper is added twice using `add_subdirectory()`.