ci: bump Node 20 actions to Node 24 versions

[thedavidmeister]

GHA forces Node 24 on runners from 2026-06-02 and removes Node 20 on 2026-09-16. Bumps the two pinned actions across all rainix workflows.

• actions/checkout@v4 -> @v6
• nix-community/cache-nix-action@v6 -> @v7

Closes #116. flake.nix nodejs_22 bump deferred (need to verify graph-cli compatibility separately).

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📥 Commits

Reviewing files that changed from the base of the PR and between d444279 and 0c6e87a.

📒 Files selected for processing (12)

• .github/workflows/check-shell.yml
• .github/workflows/publish-soldeer.yaml
• .github/workflows/rainix-build-pointers.yaml
• .github/workflows/rainix-manual-sol-artifacts.yaml
• .github/workflows/rainix-rs-static.yaml
• .github/workflows/rainix-rs-test.yaml
• .github/workflows/rainix-rs-wasm-test.yaml
• .github/workflows/rainix-rs-wasm.yaml
• .github/workflows/rainix-sol-legal.yaml
• .github/workflows/rainix-sol-static.yaml
• .github/workflows/rainix-sol-test.yaml
• .github/workflows/test.yml

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• Create PR with unit tests
• Commit unit tests in branch 2026-05-23-node24-action-pins

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