iosevka: 1.4.2 -> 1.11.4

1.11.4 contains numerous improvements including ligatures and fixes to
font weights.

I switched "default.nix" from using `fetchFromGitHub` to simply pulling
down the release as a zip via `fetchurl`. This seems like the best
approach given that the repositoriy no longer contains font files and
the alternative would be to build them from scratch.