Update horizon-flake-parts to 0.18.0
Summary
- Update horizon-flake-parts 0.16.0 → 0.18.0
- Adopt named-arg mkFlake API
- Add ci/nix/ci.nix for CI partition with horizon-ci + cachix
- Remove horizon-gen-nix (removed in 0.17.0)
Ref: nix/horizon-flake-parts#7 (closed)
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