steamPackages: add update-runtime-shell.nix to update runtime

pkgs/games/steam/update-runtime-shell.nix

···
+with import <nixpkgs> {};
+
+(python2.buildEnv.override {
+  extraLibs = with python2Packages;
+    [ debian
+    ];
+  postBuild = ''
+    mkdir -p $out/bin
+    for i in ${nixUnstable}/bin/*; do
+      ln -s $i $out/bin/$(basename $i)
+    done
+  '';
+}).env