Updated PyPI release process

[njroussel]

This PR supersedes https://github.com/mitsuba-renderer/mitsuba3/pull/1329, which originally made the following changes

• Use native arm64 Github runner and transfer CIBW envvars to pyproject.toml
• Build for also Python 3.13, stable ABI Python 3.12
• Make sure we install generated stubs
• Gracefully handle static initialization of Mitsuba for stub generation when LLVM/CUDA is missing

in this PR we additionally have the following:

• Nanobind is built as a shared library
• Wheel repair (on Linux) and testing is simplified
• The set of variants built for PyPI is updated (see below)
• Minor configuration changes for depedency requirements

The new list of variants that will be available on PyPI:

• scalar_rgb
• scalar_spectral
• scalar_spectral_polarized
• llvm_ad_rgb
• llvm_ad_mono
• llvm_ad_mono_polarized
• llvm_ad_spectral
• llvm_ad_spectral_polarized
• cuda_ad_rgb
• cuda_ad_mono
• cuda_ad_mono_polarized
• cuda_ad_spectral
• cuda_ad_spectral_polarized