steam: fix steamrt trying to use a missing setup.sh (#59326)

I was getting the following error after a Steam update:

    internal error: /steamrt/setup.sh is missing, this runtime is invalid or corrupted

The script is used to put things on PATH but it doesn't seem we need to do that.

pkgs/games/steam/chrootenv.nix

@@ -35,6 +35,10 @@ let
   ldPath = map (x: "/steamrt/${steam-runtime-wrapped.arch}/" + x) steam-runtime-wrapped.libs
            ++ lib.optionals (steam-runtime-wrapped-i686 != null) (map (x: "/steamrt/${steam-runtime-wrapped-i686.arch}/" + x) steam-runtime-wrapped-i686.libs);
 
+  setupSh = writeScript "setup.sh" ''
+    #!${runtimeShell}
+  '';
+
   runSh = writeScript "run.sh" ''
     #!${runtimeShell}
     runtime_paths="${lib.concatStringsSep ":" ldPath}"
@@ -168,6 +172,7 @@ in buildFHSUserEnv rec {
       ln -s ../lib32/steam-runtime steamrt/${steam-runtime-wrapped-i686.arch}
     ''}
     ln -s ${runSh} steamrt/run.sh
+    ln -s ${setupSh} steamrt/setup.sh
   '' else ''
     ln -s /usr/lib/libbz2.so usr/lib/libbz2.so.1.0
     ${lib.optionalString (steam-runtime-wrapped-i686 != null) ''