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/andika/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchzip}:
+{ lib, fetchzip }:
 
 let
   version = "5.000";
@@ -16,7 +16,7 @@ in
 
     sha256 = "1jy9vpcprpd1k48p20wh6jhyn909ibia8lr5i747p41l0s8a7lqy";
 
-    meta = with stdenv.lib; {
+    meta = with lib; {
       homepage = https://software.sil.org/andika;
       description = "A family designed especially for literacy use taking into account the needs of beginning readers";
       longDescription = ''