feat(nix): support selecting model targets in Nix builds

[francescobozzo]

Changes

• package.nix: Added models parameter: non-empty list auto-sets AUDIOCPP_MODEL_SET=custom, empty defaults to full
• CMakeLists.txt: EXCLUDE_FROM_ALL on all model OBJECT libraries so unselected models aren't compiled; vibevoice_finetune_overlay_check now gated on vibevoice being linked. Note: this change influences all other build pipelines, but was necessary to exclude unwanted models not just from linking but also from compile time.
• docs/build/nixos.md: New Model Selection section with CLI and NixOS examples

Tests

Validated model list reduces the number of building steps in cmake with the following configuration:

vulkan = base.override {
  vulkanSupport = true;
  models = [
    "vibevoice"
    "miotts"
  ];
};

[0xShug0]

@francescobozzo Merged! Thanks!