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/material-icons/default.nix

@@ -1,22 +1,22 @@
-{ stdenv, fetchFromGitHub }:
+{ lib, fetchFromGitHub }:
 
-stdenv.mkDerivation rec {
-  name = "material-icons-${version}";
+let
   version = "3.0.1";
+in fetchFromGitHub {
+  name = "material-icons-${version}";
 
-  src = fetchFromGitHub {
-    owner  = "google";
-    repo   = "material-design-icons";
-    rev    = "${version}";
-    sha256 = "17q5brcqyyc8gbjdgpv38p89s60cwxjlwy2ljnrvas5cj0s62np0";
-  };
+  owner  = "google";
+  repo   = "material-design-icons";
+  rev    = version;
 
-  buildCommand = ''
+  postFetch = ''
+    tar xf $downloadedFile --strip=1
     mkdir -p $out/share/fonts/truetype
-    cp $src/iconfont/*.ttf $out/share/fonts/truetype
+    cp iconfont/*.ttf $out/share/fonts/truetype
   '';
+  sha256 = "1syy6v941lb8nqxhdf7mfx28v05lwrfnq53r3c1ym13x05l9kchp";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     description = "System status icons by Google, featuring material design";
     homepage = https://material.io/icons;
     license = licenses.asl20;