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/gentium-book-basic/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchzip }:
+{ lib, fetchzip }:
 
 let
   major = "1";
@@ -11,14 +11,13 @@ in fetchzip rec {
 
   postFetch = ''
     mkdir -p $out/share/{doc,fonts}
-    unzip -l $downloadedFile
     unzip -j $downloadedFile \*.ttf                            -d $out/share/fonts/truetype
     unzip -j $downloadedFile \*/FONTLOG.txt \*/GENTIUM-FAQ.txt -d $out/share/doc/${name}
   '';
 
   sha256 = "0598zr5f7d6ll48pbfbmmkrybhhdks9b2g3m2g67wm40070ffzmd";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = https://software.sil.org/gentium/;
     description = "A high-quality typeface family for Latin, Cyrillic, and Greek";
     maintainers = with maintainers; [ ];