CI: replace deprecated `DeterminateSystems/nix-installer-action` with org-standard nix-quick-install

[thedavidmeister]

Modernization debt: deprecated Nix installer in CI

roh-scan (the org health check) flags this repo with the old-nix-installer signal: one or more GitHub Actions workflows install Nix via the deprecated DeterminateSystems/nix-installer-action.

Where

• .github/workflows/manual-sol-artifacts.yaml line 26 (@v4)

Why this is debt

The org standard for installing Nix in CI is now nix-quick-install (nixbuild/nix-quick-install-action), delivered through the shared rainix composite rainlanguage/rainix/.github/actions/nix-cachix-setup@main — a single source of truth that bundles checkout + nix-quick-install + Cachix + cache-nix-action and pins every third-party action to an exact SHA. DeterminateSystems/nix-installer-action is no longer the org's installer, and per-repo installer steps drift from the shared setup ("rainix owns shared CI").

Fix

Replace the DeterminateSystems/nix-installer-action step(s) with the org-standard install:

• Preferred: adopt rainlanguage/rainix/.github/actions/nix-cachix-setup@main where the workflow's shape allows (this also gets you Cachix + Nix-store caching for free).
• Minimum: swap the installer step for nixbuild/nix-quick-install-action pinned to the same SHA the composite uses.

Verify CI stays green after the swap.

Detected by roh-scan (old-nix-installer); see the org health dashboard.