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/overpass/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchzip }:
+{ lib, fetchzip }:
 
 let
   version = "3.0.3";
@@ -14,7 +14,7 @@ in fetchzip rec {
 
   sha256 = "1m6p7rrlyqikjvypp4698sn0lp3a4z0z5al4swblfhg8qaxzv5pg";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = http://overpassfont.org/;
     description = "Font heavily inspired by Highway Gothic";
     license = licenses.ofl;