Release v3.12.0 (#1691)

* Prepare release v3.12.0

* Update test fixtures with the new nix-installer version

* Update Determinate Nix release to 3.12.0

* flake.lock: Update

Flake lock file updates:

• Updated input 'determinate':
    'https://api.flakehub.com/f/pinned/DeterminateSystems/determinate/3.11.3/0199ca21-1b70-7110-891b-829db1f1e850/source.tar.gz?narHash=sha256-h/JQLcAfRNAo3QSobPxzTY/KxSwEmwmFJmUd5dGchQw%3D' (2025-10-09)
  → 'https://api.flakehub.com/f/pinned/DeterminateSystems/determinate/3.12.0/019a12c8-c95c-7c68-8da4-d8cc92608fbf/source.tar.gz?narHash=sha256-I/TDYHCKui0K62f2cEk2UJf6N9rO/hdsa65kpEJMhSo%3D' (2025-10-23)
• Updated input 'determinate/determinate-nixd-aarch64-darwin':
    'https://install.determinate.systems/determinate-nixd/tag/v3.11.3/macOS?narHash=sha256-Nug8jpanW4BbUfzq508ZWtCG24YGvyO8AOeTAyDMOKQ%3D'
  → 'https://install.determinate.systems/determinate-nixd/tag/v3.12.0/macOS?narHash=sha256-TORlljq%2Bwwn8XWLoN0giLY15pNiIAXuU0igpIXjLhMY%3D'
• Updated input 'determinate/determinate-nixd-aarch64-linux':
    'https://install.determinate.systems/determinate-nixd/tag/v3.11.3/aarch64-linux?narHash=sha256-kRAuNurPaJBhuOq1SpcCBmDOqCXvGDMKuMAb5vJRqtk%3D'
  → 'https://install.determinate.systems/determinate-nixd/tag/v3.12.0/aarch64-linux?narHash=sha256-1HEvUQcG0mVdEQrEqcLEdB9nHpMNbb39bdNxdvyizqk%3D'
• Updated input 'determinate/determinate-nixd-x86_64-linux':
    'https://install.determinate.systems/determinate-nixd/tag/v3.11.3/x86_64-linux?narHash=sha256-7AF8O33I/xlMengDU6tHvmPVvXJZF7XQEdAbjfTPJ6s%3D'
  → 'https://install.determinate.systems/determinate-nixd/tag/v3.12.0/x86_64-linux?narHash=sha256-WrXQbrXVisAdZl/hh49PsErSPHwzks1Vw%2BO3jarVjDo%3D'
• Updated input 'nix':
    'https://api.flakehub.com/f/pinned/DeterminateSystems/nix-src/3.11.3/0199c9f4-3aa1-723c-a5e0-be7fb957ff9f/source.tar.gz?narHash=sha256-1aMQb%2BeSrGDbTrX7PvtpD142CniIvKFxej9hZxBWeMY%3D' (2025-10-09)
  → 'https://api.flakehub.com/f/pinned/DeterminateSystems/nix-src/3.12.0/019a1277-d4c6-7dca-9d55-ee5165fd0bf6/source.tar.gz?narHash=sha256-BvEZ31%2BFQKJz2XH8PTXpJqGZ1eT9bhMQ2wBj2ehBYvM%3D' (2025-10-23)

* Update Cargo dependencies

* Set Cargo.toml package.version to 3.12.0

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]