This adds the ability to select a specific prebuilt variant. It also adds
an updater script for generating their hashes. Additionally, switching
to TTC files reduces the package size by an order of magnitude.
Example usage:
fonts.fonts = with pkgs; [
(iosevka-bin.override { variant = "ss10"; })
(iosevka-bin.override { variant = "sparkle"; })
(iosevka-bin.override { variant = "aile"; })
];
