nix: cleanup flake and add systems input

[matt1432]

This PR does the following:

• updates the Hyprland input, since it doesn't need submodules anymore with nix
• adds the systems input to allow overriding systems by end users
• bumps gcc to version 14 since that is what's used by Hyprland
• misc style changes

[KZDKM]

Thanks as always! (starting to consider switching to nix myself lmao)

[matt1432]

You really should! I absolutely love it