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/ttf-envy-code-r/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchzip }:
+{ lib, fetchzip }:
 
 let
   pname = "ttf-envy-code-r";
@@ -16,11 +16,10 @@ in fetchzip {
 
   sha256 = "0x0r07nax68cmz7490x2crzzgdg4j8fg63wppcmjqm0230bggq2z";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = http://damieng.com/blog/tag/envy-code-r;
     description = "Free scalable coding font by DamienG";
     license = licenses.unfree;
-    platforms = platforms.unix;
     maintainers = [ maintainers.lyt ];
   };
 }