[CI] Enable parallel GitHub Actions wheel builds

[tlopex]

This PR enables parallel TVM wheel builds on macOS and Windows GitHub Actions by detecting the available CPU count and passing it to CMake through CMAKE_BUILD_PARALLEL_LEVEL.

[gemini-code-assist]

Code Review

This pull request introduces a new GitHub composite action, detect-env-vars, which runs a Python script to detect the system's CPU count and export it as an output. The review feedback suggests making the Python script more robust by using a with statement to ensure the output file is properly closed, checking for the existence of the GITHUB_OUTPUT environment variable to prevent a TypeError, and using os.cpu_count() with a fallback to avoid potential NotImplementedError exceptions.

Important

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