Update Nix version requirement from 2.13 to 2.18 and switch to nixbuild/nix-quick-install-action@v32

[Copilot]

This PR updates the minimum Nix version requirement from 2.13 to 2.18 to address CI failures caused by nixpkgs requiring a newer Nix version, and switches all GitHub Actions workflows to use nixbuild/nix-quick-install-action@v32 for better performance.

Changes Made

Core Version Update:

• Updated the version assertion in flake.nix from 2.13 to 2.18
• Updated the corresponding error message to reflect the new minimum version requirement

CI Workflow Updates:

• Replaced DeterminateSystems/nix-installer-action@main with nixbuild/nix-quick-install-action@v32 in all workflows
• Updated .github/workflows/std.yml (4 instances across discover, cli, shells, and tests jobs)
• Updated .github/workflows/gh-pages.yaml
• Updated .github/workflows/flakehub-publish-tagged.yml

Context

The CI was failing because the current nixpkgs version requires Nix 2.18 or higher, but the Standard framework was still enforcing a minimum requirement of 2.13. This mismatch caused build failures in the GitHub Actions workflows.

The workflow updates use nixbuild/nix-quick-install-action@v32 which installs Nix 2.18+ that meets the new requirement while providing faster installation times and maintaining cross-platform compatibility for both Linux and macOS runners.

Fixes #411.

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