stylix: re-add `flake-utils` dependency

[trueNAHO]

To: @danth

Why was the flake-utils dependency removed in commit ff5da2914cfc ("Remove dependency on flake-utils :heavy_minus_sign:")?

Some of its functionality, like handling the --impure flag, are neat but would be too costly to re-implement on our side.

Re-adding flake-utils and injecting the "i686-linux" system would resolve this issue:

• https://github.com/danth/stylix/blob/94d70292d0c687ebacb65d00bd516cbefa18d3ca/flake.nix#L61-L67

• defaultSystems :: [<system>]

  The list of systems to use in eachDefaultSystem and simpleFlake.

  The default values are ["x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin"].

  -- https://github.com/numtide/flake-utils/blob/b1d9ab70662946ef0850d488da1c9019f3a9752a/README.md

[trueNAHO]

I already have a working PR. Once it is cleaned up I can submit it for review (tomorrow).

Coincidentally, the PR publicly interfaces an overridable list of supported architectures.

[danth]

I didn't think it was worth the extra dependency for something which could be implemented fairly easily using functions from the nixpkgs library.

Admittedly things have changed in both Stylix and flake-utils since that commit; there may also be additions to the nixpkgs library which could be useful here.