Evaluate with aliases enabled additionally

[sternenseemann]

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https://github.com/NixOS/nixpkgs/pull/349431/ https://github.com/NixOS/nixpkgs/pull/349783 show that changes to aliases can break evaluation of nixpkgs with aliases enabled while not affecting it with aliases disabled. Almost all downstream users will probably have aliases enabled, so this is a situation we should ideally check for. Saving grace is probably that most Hydra jobsets have aliases enabled.