ci: adopt rainix nix-cachix-setup composite in manual-sol-artifacts, drop deprecated DeterminateSystems nix installer

[thedavidmeister]

Closes #484

What

Replaces the deprecated DeterminateSystems/nix-installer-action@v4 (+ magic-nix-cache-action@v2) in .github/workflows/manual-sol-artifacts.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 step (with submodules: recursive + fetch-depth: 0) is kept, so the composite is called with checkout: 'false'. The two nix develop --command rainix-sol-{prelude,artifacts} deploy steps and all deploy secrets/env (ETH_RPC_URL, ETHERSCAN_API_KEY, DEPLOYMENT_KEY) are untouched.

This is the same swap already applied and green on the sibling PRs rainlanguage/sqlite-web#31, rainlanguage/rainlang-codemirror#32, rainlanguage/rain.subgraph.docker#13, and rainlanguage/rain.webapp#357.

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 — the oracle is the workflow run itself.

• Weaker oracle noted honestly: manual-sol-artifacts.yaml is workflow_dispatch-triggered (a manual per-network deploy), so this PR's push does not auto-exercise the swap the way an on: push CI workflow would. The equivalence argument below carries the confidence instead.
• Independent confirmation: the identical installer + magic-nix-cache → composite swap is already green on the sibling PRs above; the composite's input contract (checkout, cachix-auth-token) matches nix-cachix-setup/action.yml on rainix@main, and this deploy workflow's shape (single checkout + installer, no custom extra-conf) is exactly the shape the composite supports.
• Equivalence: the composite provides the same capabilities the two removed steps did (Nix install + a Nix store cache) plus the org's pinned-SHA hardening. checkout: 'false' preserves the existing actions/checkout@v4 (submodules + full depth). An empty CACHIX_AUTH_TOKEN degrades to a read-only Cachix pull (the composite's documented default). The deploy run steps and all network-selecting secrets are unchanged, so deploy behavior is identical — only the Nix install mechanism moves to the org standard.
• Category check: issue asks to replace the deprecated installer with the org-standard install (preferred: adopt the shared composite) at the one referenced location and verify CI stays green — done → Closes CI: replace deprecated DeterminateSystems/nix-installer-action with org-standard nix-quick-install #484.

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

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📥 Commits

Reviewing files that changed from the base of the PR and between 9ce782c and 12eb999.

📒 Files selected for processing (1)

• .github/workflows/manual-sol-artifacts.yaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/adopt-nix-cachix-setup-484

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[thedavidmeister]

🤖 ai:vetter
Reviewed 12eb999: ready — closes #484 — adopts org-standard nix-cachix-setup@main composite (checkout:false keeps existing checkout, cachix-auth-token wired) replacing deprecated DeterminateSystems installer+magic-nix-cache in manual-sol-artifacts.yaml, the sole occurrence repo-wide; deploy steps/secrets untouched; matches already-vetted sibling swaps
cost 183 — CI cachix composite adoption, single workflow