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/seshat/default.nix

@@ -1,4 +1,4 @@
-{ stdenv,  fetchzip }:
+{ lib,  fetchzip }:
 
 let
   majorVersion = "0";
@@ -17,7 +17,7 @@ fetchzip rec {
     unzip -j $downloadedFile \*.otf  -d $out/share/fonts/opentype/${pname}
   '';
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = "http://dotcolon.net/font/${pname}/";
     description = "Roman body font designed for main text by Sora Sagano";
     longDescription = ''