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/montserrat/default.nix

@@ -2,7 +2,7 @@
 #
 # https://aur.archlinux.org/packages/ttf-montserrat/
 
-{ stdenv, fetchzip }:
+{ lib, fetchzip }:
 
 let
   version = "1.0";
@@ -19,7 +19,7 @@ in fetchzip {
 
   sha256 = "11sdgvhaqg59mq71aqwqp2mb428984hjxy7hd1vasia9kgk8259w";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     description = "A geometric sans serif font with extended latin support (Regular, Alternates, Subrayada)";
     homepage    = "https://www.fontspace.com/julieta-ulanovsky/montserrat";
     license     = licenses.ofl;