Add default omnix package to modules

Add default omnix package to modules

Both omnix and cachix modules now have a package/omnix option that defaults to omnix from gitlab-ci's nixpkgs (nixos-25.05).

This allows the modules to work on older nixpkgs versions (like those used by GHC 8.10) that don't have omnix available.

Changes

• Add pkgsLocal helper function to import gitlab-ci's nixpkgs
• Add package option to omnix module (defaults to pkgsLocal.omnix)
• Add omnix option to cachix module (defaults to pkgsLocal.omnix)
• Both modules now accept system as an argument to instantiate the correct nixpkgs

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