EPIC: Add Nix / nixpkgs support for CUDA Python

[rparolin]

Summary

Scope and land first-class support for installing and using cuda-python
(and its sub-packages: cuda.bindings, cuda.core, cuda.pathfinder) via
the Nix package manager / nixpkgs.

Today there is no tracked story for Nix users. This EPIC is a placeholder
for the team to break down and scope the work.

Motivation

• Nix/NixOS users currently have to roll their own derivations to consume
  cuda-python alongside the CUDA Toolkit that nixpkgs already ships
  (see https://search.nixos.org/packages).
• A reproducible, upstream-blessed path lowers friction for reproducible
  research, HPC, and CI environments that standardize on Nix.
• Aligns with the cuda.pathfinder direction of supporting many install
  layouts (EPIC: Path finder for CUDA components #451) — Nix store paths are yet another layout to locate
  headers/libs from.

Open questions / areas to scope

• How can we align Nix work with CCCLs effort?
• How does cuda.pathfinder locate components when CUDA lives under
  /nix/store/...-cudatoolkit-*/? Confirm discovery works or add a
  Nix-aware search path (ties to EPIC: Path finder for CUDA components #451).
• Cython build requirements for cuda.bindings under Nix (CUDA version
  pinning, stub libraries, LD_LIBRARY_PATH vs rpath patching).
• CI: add a Nix build job to catch regressions.
• Documentation: install instructions for Nix/NixOS users.

References

• nixpkgs CUDA package search: https://search.nixos.org/packages?query=cuda

[rparolin]

I created this issue in error. Closing the ticket.