Update to gitlab-ci 0.8.0 and dhall spec 0.18.0

Summary

Updates the Haskell horizon-spec library to:

• Use gitlab-ci 0.8.0 for CI configuration
• Support dhall spec 0.18.0 with simplified HaskellPackage type

Changes

CI Updates

• Migrate from flake-to-gitlab-ci to modular gitlab-ci 0.8.0
• Add horizon-supported-systems for system management
• Update nix/checks.nix to use 0.18.0 dhall spec dependencies

Type Updates (0.18.0)

• HaskellPackage simplified to only source and flags fields
• Removed policy and patches fields (moved to export settings)
• Test data updated from 0.17.0 to 0.18.0
• Updated horizon-spec.dhall references to 0.18.0

Library Version

• Library version: 0.13.0
• Test suite version: 0.13.0

Dependencies

Depends on dhall/horizon-spec!18 (merged) being merged first (0.18.0 release).

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