ci: disable Determinate Nix job summaries

[SDAChess]

Summary

Disable Determinate Nix build summaries and timing charts to reduce noise in GitHub Actions reports.

Related Issue

None, as requested by the maintainer.

Changes

• Set summarize: false in the shared Nix setup action.

Testing

• YAML parsed and summarize: false verified with yq through nix develop.
• git diff --cached --check passed.
• Pre-commit hook skipped with explicit approval after unrelated Python/Node tool installation failed.
• Unit and E2E tests are not applicable to this configuration change.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updates are not applicable.