steamPackages: add runtime-mirror helper derivation

pkgs/games/steam/runtime-mirror.nix

@@ -0,0 +1,20 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  inherit (pkgs) lib;
+  rt = import ./runtime-generated.nix { inherit (pkgs) fetchurl; };
+  convRt = x: {
+    path = lib.removePrefix "mirror://steamrt/" x.url;
+    file = x.source;
+  };
+  files = builtins.map convRt (lib.concatLists (lib.attrValues rt));
+  
+in pkgs.stdenv.mkDerivation {
+  name = "steam-runtime-mirror";
+  buildCommand = ''
+    mkdir $out
+  '' + lib.concatMapStringsSep "\n" (x: ''
+    mkdir -p $(dirname $out/${x.path})
+    ln -sf ${x.file} $out/${x.path}
+  '') files;
+}