Use UV to build Python wheels

[oleksandr-pavlyk]

1. This follows the lead of similar change on CCCL side: Switch from pyenv to uv cccl#8398
2. Transitioning away from using pyenv allows avoiding the need to build CPython on every pull request, cutting down build time.

Closes #399

[oleksandr-pavlyk]

@coderabbitai full review

[coderabbitai]

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved CUDA wheel build reliability by using the intended Python version throughout merge and repair steps.
  • Standardized wheel and repair commands to run in the prepared environment, reducing mismatches during CI builds.

• Chores

  • Updated CI Python environment setup to use a more consistent temporary virtual environment flow.
  • Added a version check to fail fast if the prepared environment does not match the requested Python version.

Walkthrough

ci/pyenv_helper.sh replaces the pyenv/shim-based CPython build with a uv-based virtual environment setup. ci/build_multi_cuda_wheel.sh sources the updated helper, calls setup_python_env, and replaces all $PYTHON references with plain python for wheel and auditwheel operations.

Changes

pyenv to uv migration

Layer / File(s)	Summary
uv-based venv setup ci/pyenv_helper.sh	setup_python_env() detects/installs uv, creates a venv at NVBENCH_PYTHON_VENV, activates it (Windows or POSIX path), and exits with an error if the interpreter major/minor version does not match ${py_version}.
build script wired to new helper ci/build_multi_cuda_wheel.sh	Sources ci/pyenv_helper.sh, calls setup_python_env "${py_version}", installs wheel via python -m pip, and replaces all $PYTHON -m wheel/$PYTHON -m auditwheel calls with python -m ..., removing --break-system-packages.

Assessment against linked issues

Objective	Addressed	Explanation
Replace pyenv CPython build with uv [#399]	✅

Out-of-scope changes

No out-of-scope changes found.

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