Add CUDA wheels as build-time requirements for build isolation

[fheinecke]

Description

One of the issues blocking wheel build isolation is that CUDA build-time dependencies (e.g. nvcc) are not available today. These cannot easily be statically defined in pyproject.toml, because the exact dependencies required depend on whether the wheel is being built for CUDA 12 or CUDA 13.

This PR adds a PEP 517/660 compliant build_system.py that defines these requirements based on specific build-time flag, env var, or system dependencies. As with today, this logic is completely ignored when --no-build-isolation is set, so it should have no impact on existing release processes. This does not completely enable build isolation, but fixes most of the remaining blockers.

I have intentionally listed this as "not for external use" in the documentation. I'll update this and the install docs once build isolation is completely supported.

Depends on #3251

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Enables installing CUDA version-specific, build-time dependencies via wheels

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[fheinecke]

/te-ci