nix flake refactor: avoid `with lib;` & use `lib.optional*`

[toastal]

Upstream Nixpkgs has been pushing the style of removing with lib; with the reasoning of clarity around where variables in scope come that hurt readability & static analysis^1.

While on the topic of lib, introduce lib.optional & lib.optionalString in a few more places to tidy up the code while using common library patterns.

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Also has been open @ https://gitlab.com/StevenBlack/hosts/-/merge_requests/2

[benjamb]

Looks good to me! I tested on my local system as well, to ensure the build succeeded with these changes.

[Pandapip1]

Also bump!