Test refactoring: factor out `macli` tests to nix/checks/macli.nix Prior to this change, the check derivations were defined in nix/checks.nix but the actual tests depended on functions and lambdas defined in flake.nix. This change makes the `macli` checks more self-contained and moves them to nix/checks/macli.nix. Common Nix code that is used in flake.nix and macli.nix has been moved to nix/common.nix.