[BUG]: thrust/execution_policy.h is not safe to include without cuda runtime available

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Type of Bug

Compile-time Error

### Component

Thrust

### Describe the bug

Including the execution_policy.h file in a TU being compiled by a host compiler without the CUDA runtime libraries present triggers an immediate compile error:
```
In file included from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/system/cuda/config.h:48,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/system/cuda/detail/execution_policy.h:40,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/iterator/detail/device_system_tag.h:31,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/iterator/iterator_traits.h:74,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/detail/type_traits/pointer_traits.h:31,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/detail/allocator/allocator_traits.h:37,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/detail/execute_with_allocator.h:29,
from /home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/thrust/../../../thrust/thrust/execution_policy.h:32,
from /home/vyasr/local/cccl/examples/basic/example.cpp:1:
/home/vyasr/local/cccl/examples/basic/build/_deps/cccl-src/lib/cmake/cub/../../../cub/cub/detail/detect_cuda_runtime.cuh:49:12: fatal error: cuda_runtime_api.h: No such file or direct
ory
49 | # include
```

### How to Reproduce

With this CMake:
```
cmake_minimum_required(VERSION 3.26.1 FATAL_ERROR)
project(ExecutionPolicyError CXX)
file(DOWNLOAD
https://github.com/cpm-cmake/CPM.cmake/releases/download/v0.40.2/CPM.cmake
${CMAKE_CURRENT_BINARY_DIR}/CPM.cmake LOG download_log)
include(${CMAKE_CURRENT_BINARY_DIR}/CPM.cmake)
CPMAddPackage(
NAME CCCL
GITHUB_REPOSITORY nvidia/cccl
GIT_TAG main
)
add_executable(example_project example.cpp)
target_link_libraries(example_project PRIVATE CCCL::CCCL)
```

and this C++ file
```
#include
```

running `cmake -S . -B build/ && cmake --build build/` is sufficient to trigger the compilation error.

### Expected behavior

Thrust should be safe to include in host TUs without the CUDA libraries available.

### Reproduction link

_No response_

### Operating System

Ubuntu Linux 24.04

### nvidia-smi output

N/A (not using GPU)

### NVCC version

N/A (not using nvcc)