Update default nix_package_url to Nix 2.14.1

[cole-h]

Description

2.14.0 was released today.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

---

Drafted until NixOS/nix#7920 NixOS/nix#7925 is resolved, as that could break some CI machines.

[Hoverbear]

I restarted the buildkite jobs that failed

[drupol]

2.14.1 has been released in the meantime: https://github.com/NixOS/nix/releases/tag/2.14.1

[cole-h]

I'm personally waiting on seeing how NixOS/nix#7984 shakes out before marking this as ready-for-review. Despite being flakes-first, there are still people who use the legacy CLI with this installer (i.e. #234). I know that at the very least nix-env -iA nixpkgs.hello is broken as of 2.14.1 directly after a multi-user install (though this can be fixed with a couple nix-channel invocations, it's difficult to do that automatically since users don't fall back to root's channels anymore it seems).

[cole-h]

Note to self: make sure #304 is still fine after we bump to 2.14.