top-level: Make stdenvCross which appears at first glance normal...

...but actually is weird just like the original

pkgs/top-level/default.nix

@@ -12,6 +12,11 @@
   # null, the default standard environment is used.
   bootStdenv ? null
 
+, # This is used because stdenv replacement and the stdenvCross do benefit from
+  # the overridden configuration provided by the user, as opposed to the normal
+  # bootstrapping stdenvs.
+  allowCustomOverrides ? (bootStdenv == null)
+
 , # Non-GNU/Linux OSes are currently "impure" platforms, with their libc
   # outside of the store.  Thus, GCC, GFortran, & co. must always look for
   # files in standard system directories (/usr/include, etc.)
@@ -109,7 +114,7 @@ let
   # attributes to refer to the original attributes (e.g. "foo =
   # ... pkgs.foo ...").
   configOverrides = self: super:
-    lib.optionalAttrs (bootStdenv == null)
+    lib.optionalAttrs allowCustomOverrides
       ((config.packageOverrides or (super: {})) super);
 
   # The complete chain of package set builders, applied from top to bottom