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/gandom-fonts/default.nix

@@ -1,22 +1,21 @@
-{ stdenv, fetchFromGitHub }:
+{ lib, fetchFromGitHub }:
 
-stdenv.mkDerivation rec {
+let
   pname = "gandom-fonts";
   version = "0.6";
+in fetchFromGitHub {
+  name = "${pname}-${version}";
+  owner = "rastikerdar";
+  repo = "gandom-font";
+  rev = "v${version}";
 
-  src = fetchFromGitHub {
-    owner = "rastikerdar";
-    repo = "gandom-font";
-    rev = "v${version}";
-    sha256 = "1pdbqhvcsz6aq3qgarhfd05ip0wmh7bxqkmxrwa0kgxsly6zxz9x";
-  };
-
-  installPhase = ''
-    mkdir -p $out/share/fonts/gandom-fonts
-    cp -v $( find . -name '*.ttf') $out/share/fonts/gandom-fonts
+  postFetch = ''
+    tar xf $downloadedFile --strip=1
+    find . -name '*.ttf' -exec install -m444 -Dt $out/share/fonts/gandom-fonts {} \;
   '';
+  sha256 = "0zsq6s9ziyb5jz0v8aj00dlxd1aly0ibxgszd05dfvykmgz051lc";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = https://github.com/rastikerdar/gandom-font;
     description = "A Persian (Farsi) Font - فونت (قلم) فارسی گندم";
     license = licenses.ofl;