Enable provenance in CI

[lucperkins]

Summary by CodeRabbit

• Chores

  • Updated CI checkout action to the latest version across workflows.
  • Added provenance support to Nix build steps and standardized Nix installer tooling.
  • Switched formatter selection in project config.

• Documentation

  • Updated workflow docs and embedded examples to reflect new action versions and configuration.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

Upgrades GitHub Actions checkout usages to v6 across workflows and embedded examples, adds extra-conf with extra-experimental-features = provenance to several determinate-nix-action invocations in CI, replaces nix-installer-action references with determinate-nix-action, and switches a flake formatter reference to pkgs.nixfmt.

Changes

Cohort / File(s)	Summary
CI workflow .github/workflows/ci.yaml	Upgraded actions/checkout references to @v6. Added `with: extra-conf:
Update workflow .github/workflows/update.yml	Changed uses: actions/checkout@v4 → uses: actions/checkout@v6.
Documentation / README README.md	Replaced checkout action versions with v6; replaced nix-installer-action@main with determinate-nix-action@main and updated step names and example snippet labels.
Source example src/github/mod.rs	Updated embedded workflow example actions/checkout reference from @v3 → @v6. Minor comment formatting tweak.
Nix flake flake.nix	Switched formatter binding from pkgs.nixfmt-rfc-style to pkgs.nixfmt in the overlay outputs.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰
I hopped through workflows, swift and spry,
Swapping checkouts so v6 can fly,
Nix wears provenance like morning dew,
Formatter trimmed, examples too.
Cheers — a rabbit's small deploying cry! 🥕

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Enable provenance in CI' directly aligns with the primary change: adding provenance configuration to determinate-nix-action steps across multiple CI workflows.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch enable-provenance

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