ci: adopt rainix nix-cachix-setup composite, drop deprecated DeterminateSystems nix installer

[thedavidmeister]

Closes #30

What

Replaces the deprecated DeterminateSystems/nix-installer-action@main (+ flakehub-cache-action@main) in .github/workflows/npm-release.yaml with the org-standard shared composite rainlanguage/rainix/.github/actions/nix-cachix-setup@main, which bundles nix-quick-install + Cachix + cache-nix-action and pins every third-party action to an exact SHA (single source of truth — "rainix owns shared CI"). The pre-existing actions/checkout@v4 (with ssh-key, submodules recursive, fetch-depth 0) is kept, so the composite is called with checkout: 'false'.

This also removes the @main floating-ref supply-chain risk the roh-scan signal called out (the composite pins every third-party action to an exact SHA). test-ui.yaml/test-wasm.yaml install no Nix and are untouched.

This is the same swap already applied and green on the pilot PRs rainlanguage/rain.chainlink#11, rainlanguage/rain.tier.interface#8, and rainlanguage/assemblyscript-cbor#5.

QA

CI-infrastructure-only change: it touches a single GitHub Actions workflow file and no source or test code, so there is no behavioral code surface to mutation-test.

• Scope honesty: npm-release.yaml runs on: release, so this PR's own CI does not exercise it — there is no PR-triggered run to observe. The change is validated by equivalence to the established pattern rather than by a PR-CI run, and I flag that explicitly here rather than implying a green run proves it.
• Independent confirmation: the identical installer → composite swap is already green on the pilots rain.chainlink#11 / rain.tier.interface#8 / assemblyscript-cbor#5; the composite's input contract (checkout, cachix-auth-token) matches nix-cachix-setup/action.yml on rainix@main.
• Equivalence: the composite provides the same capabilities the two removed steps did (Nix install + a flake/store cache) plus the org's pinned-SHA hardening. checkout: 'false' preserves the existing checkout (ssh-key: PUBLISH_PRIVATE_KEY, submodules recursive, fetch-depth 0), so the release job's SSH-authed publish path is unchanged. The subsequent setup-node and publish steps are untouched. An empty CACHIX_AUTH_TOKEN degrades to a read-only Cachix pull (the composite's documented default).
• Category check: issue asks to replace the deprecated installer (npm-release.yaml) with the org-standard install (preferred: adopt the shared composite) — done → Closes CI: replace deprecated DeterminateSystems/nix-installer-action with org-standard nix-quick-install #30.

Co-Authored-By: Claude noreply@anthropic.com

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📥 Commits

Reviewing files that changed from the base of the PR and between 9c4ed48 and 394d9c2.

📒 Files selected for processing (1)

• .github/workflows/npm-release.yaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci-adopt-nix-cachix-setup

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[thedavidmeister]

🤖 ai:vetter
Reviewed 394d9c2: ready — closes #30 — swaps deprecated DeterminateSystems installer for org nix-cachix-setup composite in npm-release.yaml; checkout:false + cachix-auth-token match the composite's real input contract, existing ssh-key deploy checkout preserved; sole roh-scan-flagged file
cost 158 — CI installer→composite swap, contract-verified