ci: simplify Nix setup

[robjtede]

Summary

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

Why

The latest CI baseline showed the cache post-step taking 73s for stable tests and 56s for MSRV tests. The stable job took 2m13s overall, while its test command took 5s. The release baseline also spent 14s in the cache post-step for a 1s changelog operation.

The focused shells retain only the capabilities each workflow needs: the shared Just shell hook 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 and coverage workflows run 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 89908c7 and 1c6b4fb.

📒 Files selected for processing (4)

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

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