Pin cuda-toolkit wheel to container's CTK major.minor in CI

[leofang]

Summary

• Pin cuda-toolkit PyPI wheels in CI to match the container's exact CTK major.minor version (e.g. cuda-toolkit==12.9.*) instead of floating to the latest (e.g. cuda-toolkit==12.*)
• Uses pip's native PIP_CONSTRAINT mechanism — no changes to pyproject.toml, so end users are unaffected
• Applies to all 4 Python test scripts: test_cuda_cccl_headers, test_cuda_compute, test_cuda_coop, test_cuda_cccl_examples

Motivation

The Python CI test scripts previously extracted only the major version from nvcc (e.g. 12) and installed cuda-toolkit==12.*, which always resolved to the latest 12.x from PyPI regardless of the container's actual CTK version. This meant that even when running in a CTK 12.0 container, CI would install cuda-toolkit 12.9 wheels (as discovered in #8139 (comment)).

This masked issues like the nvrtc compiler bug in CTK 12.4, because the pip-installed nvrtc (latest) was always used instead of the container's version.

How it works

Each test script now also extracts the X.Y version from nvcc --version and writes a pip constraint file:

cuda_version=$(nvcc --version | grep release | awk '{print $6}' | tr -d ',' | cut -d '.' -f 1-2 | cut -d 'V' -f 2)
export PIP_CONSTRAINT="${TMPDIR:-/tmp}/ctk-constraint.txt"
echo "cuda-toolkit==${cuda_version}.*" > "$PIP_CONSTRAINT"

PIP_CONSTRAINT is a standard pip environment variable that automatically constrains all subsequent pip install commands in the script.

Test plan

• Verify Python CI jobs pass with this change (the constraint should be a no-op for jobs where the container CTK matches the latest wheels)
• Ideally test with a lower-bound CTK container (e.g. 12.0) to confirm that cuda-toolkit==12.0.* is installed instead of 12.9

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[leofang]

/ok to test 7f3f7a1

[github-actions]

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