ci: simplify Nix setup

[robjtede]

Summary

• install Nix with nix-quick-install-action
• remove the FlakeHub cache from CI and release
• keep nix-develop separate
• use a focused .#ci shell for tests and .#ci-release for release tooling
• keep the full default development shell unchanged

Why

The latest CI baseline still spends setup time on Determinate Nix, lazy trees, and FlakeHub Cache for short jobs: stable took 1m05s and MSRV 1m11s, with tests taking 11s and 4s respectively. The focused shells retain only the shared Just shell hook and cargo-nextest for CI, and the x52 release tools for release.

Validation

• nix develop .#ci -c just --list
• nix develop .#ci-release -c command -v x52-bump-changelogs
• nix develop .#ci-release -c command -v x52-update-release-notes
• nix flake check --no-build --all-systems

The release workflow runs only after pushes to main; the PR CI workflow validates the quick installer path directly.

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📥 Commits

Reviewing files that changed from the base of the PR and between 3efa760 and 1c135f9.

📒 Files selected for processing (3)

• .github/workflows/ci.yml
• .github/workflows/release.yml
• flake.nix

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