fix(setup): fix a cannot found cmake issue when install from source with venv'ed cmake

[XuehaiPan]

Description

Describe your changes in detail.

Unset PYTHONPATH before calling cmake in subprocess.

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

• I have raised an issue to propose this change (required for new features and bug fixes)

See the following issues for more context:

• [BUG] optree tries to install cmake from PyPI unconditionally #187
• feat(setup): support using system CMake #188
• [BUG] Cannot find the cmake module in a wheel build venv when the cmake executable is installed in another venv scikit-build/cmake-python-distributions#586
• [BUG] Isolated build env breaks console scripts installed in other venv (e.g., cmake, ninja) pypa/pip#13222

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

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• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (acf6ab3) to head (159710b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #191   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         3987      3987           
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  Hits          3987      3987           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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