[BUG] build.sh assumes RAPIDS_CUDA_VERSION is set when run locally

[mroeschke]

Describe the bug
We document using build.sh as a way to create a local, conda, development environment in cudf.

After #23472, build.sh assumes that RAPIDS_CUDA_VERSION is set when building wheels

https://github.com/rapidsai/cudf/blob/d8fdbccd4b90b5441f9546e81420a9f56a326729/build.sh#L83

Steps/Code to reproduce bug
When not set

Obtaining file:///repos/cudf/python/pylibcudf
  Checking if build backend supports build_editable ... done
  Preparing editable metadata (pyproject.toml) ... error
  error: subprocess-exited-with-error
  
  × Preparing editable metadata (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [27 lines of output]
      Traceback (most recent call last):
        File "/conda_envs/cudf-dev/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
          main()
          ~~~~^^
        File "/conda_envs/cudf-dev/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 373, in main
          json_out["return_val"] = hook(**hook_input["kwargs"])
                                   ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
        File "/conda_envs/cudf-dev/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 209, in prepare_metadata_for_build_editable
          return hook(metadata_directory, config_settings)
        File "/conda_envs/cudf-dev/lib/python3.14/site-packages/rapids_build_backend/impls.py", line 418, in prepare_metadata_for_build_editable
          with _edit_pyproject(config):
               ~~~~~~~~~~~~~~~^^^^^^^^
        File "/conda_envs/cudf-dev/lib/python3.14/contextlib.py", line 141, in __enter__
          return next(self.gen)
        File "/conda_envs/cudf-dev/lib/python3.14/site-packages/rapids_build_backend/impls.py", line 234, in _edit_pyproject
          rapids_dependency_file_generator.make_dependency_files(
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
              parsed_config=parsed_config,
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          ...<4 lines>...
              to_stdout=False,
              ^^^^^^^^^^^^^^^^
          )
          ^
        File "/conda_envs/cudf-dev/lib/python3.14/site-packages/rapids_dependency_file_generator/_rapids_dependency_file_generator.py", line 479, in make_dependency_files
          raise ValueError(f"No matching matrix found in '{include}' for: {matrix_combo}")
      ValueError: No matching matrix found in 'depends_on_cupy' for: {'cuda': '', 'cuda_suffixed': 'false', 'use_cuda_wheels': 'false', 'arch': 'x86_64'}
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> from file:///repos/cudf/python/pylibcudf

cc @KyleFromNVIDIA