top-level: Duplicate overlaying unless stdenvOverrides comes last

The stdenvOverrides overlay is used to bring packages forward during
bootstrapping via stdenv.overrides. These packages have already had
the overlays applied to them in the previous boostrapping stage. If
stdenvOverrides is not last in the overlays stack, all remaining
overlays will windup being applied again to these packages.

closes #34086

pkgs/top-level/stage.nix

···
     lib.optionalAttrs allowCustomOverrides
       ((config.packageOverrides or (super: {})) super);
 
-  # The complete chain of package set builders, applied from top to bottom
+  # The complete chain of package set builders, applied from top to bottom.
+  # stdenvOverlays must be last as it brings package forward from the
+  # previous bootstrapping phases which have already been overlayed.
   toFix = lib.foldl' (lib.flip lib.extends) (self: {}) ([
     stdenvBootstappingAndPlatforms
     platformCompat
···
     splice
     allPackages
     aliases
-    stdenvOverrides
     configOverrides
-  ] ++ overlays);
+  ] ++ overlays ++ [
+    stdenvOverrides ]);
 
 in
   # Return the complete set of packages.