nix: remove flake-utils dep & reorganize

[toastal]

All downstream users were needing to pull in flake-utils to just call a for loop which isn’t a good UX. Inlined that loop.

Also moved steam-tui into the packages.steam-tui. Also allowlisted the nonfree-software rather than allowing it all thru.

nix build worked as well as nix develop.

[toastal]

Can be removed since inputsFrom = builtins.attrValues self.packages.${system};

[toastal]

Motivator: the nixpkgs version is woefully out of date & the latest release here (I believe) missed some fix needed for Nix last time I read the logs. I was going to pull this flake in as an overlay, but then I’d need to add flake-utils which adds more bloat to my flake.lock for that for loop. Organization things are tiny, but I tend to see devShells at the bottom and packages like steam-tui @ packages.steam-tui & aliasing that as default rather than having steam-tui as a top-level output.

[dmadisetti]

Thank you, LGTM