source-han-*: reduce closure size by 2x

The file name of the installed font was the path returned by fetchurl:
`xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-SourceHanSans.ttc`

This caused the derivation to reference the downloaded font file,
storing the font twice unless you optimise the store (each file is
~150M).

pkgs/data/fonts/source-han/default.nix

···
     version = lib.removeSuffix "R" rev;
 
     buildCommand = ''
-      install -m444 -Dt $out/share/fonts/opentype/source-han-${family} ${ttc}
+      mkdir -p $out/share/fonts/opentype/source-han-${family}
+      ln -s ${ttc} $out/share/fonts/opentype/source-han-${family}/SourceHan${Family}.ttc
     '';
 
     meta = {