data/fonts: cleanup

* make font derivations fixed-output where applicable
 * fix dead links
 * `stdenv.lib` -> `lib` where `stdenv` is not involved
 * remove `meta.platforms = [ unix ]` and `meta.platforms = [ linux ]` because the restriction has no sense for data packages

pkgs/data/fonts/mononoki/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchzip }:
+{ lib, fetchzip }:
 
 let
   version = "1.2";
@@ -14,7 +14,7 @@ in fetchzip {
 
   sha256 = "19y4xg7ilm21h9yynyrwcafdqn05zknpmmjrb37qim6p0cy2glff";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = https://github.com/madmalik/mononoki;
     description = "A font for programming and code review";
     license = licenses.ofl;