dejavu_fontsEnv: explain the situation in a comment

pkgs/top-level/all-packages.nix

···
     inherit (perlPackages) FontTTF;
   });
 
+  # solve collision for nix-env before https://github.com/NixOS/nix/pull/815
   dejavu_fontsEnv = buildEnv {
     name = "${dejavu_fonts.name}";
     paths = [ dejavu_fonts.out ];