stdenv: fix overriding with attrset when finalAttrs isn't used

```
nix-repl> (pkgs.htop.overrideAttrs { pname = "hello-overriden"; }).pname
error:
       … while evaluating a branch condition

         at /nix/store/phn5cahwacv9wjgalygw62x8l4xbl6x3-source/lib/customisation.nix:86:7:

           85|     in
           86|       if builtins.isAttrs result then
             |       ^
           87|         result // {

       … while calling the 'isAttrs' builtin

         at /nix/store/phn5cahwacv9wjgalygw62x8l4xbl6x3-source/lib/customisation.nix:86:10:

           85|     in
           86|       if builtins.isAttrs result then
             |          ^
           87|         result // {

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: attempt to call something which is not a function but a set

       at /nix/store/phn5cahwacv9wjgalygw62x8l4xbl6x3-source/pkgs/stdenv/generic/make-derivation.nix:58:21:

           57|           f = self: super:
           58|             let x = f0 super;
             |                     ^
           59|             in
```

pkgs/stdenv/generic/make-derivation.nix

@@ -62,7 +62,7 @@ let
                 f0 self super
               else x;
         in
-          makeDerivationExtensible (self: attrs // f self attrs))
+          makeDerivationExtensible (self: attrs // (if builtins.isFunction f0 || f0?__functor then f self attrs else f0)))
       attrs;
 
   mkDerivationSimple = overrideAttrs:

pkgs/test/overriding.nix

@@ -26,6 +26,11 @@ let
         expr = (pkgs.hello.overrideAttrs { pname = "hello-overriden"; }).pname == "hello-overriden";
         expected = true;
       })
+      ({
+        name = "overriding-using-only-attrset-no-final-attrs";
+        expr = ((stdenvNoCC.mkDerivation { pname = "hello-no-final-attrs"; }).overrideAttrs { pname = "hello-no-final-attrs-overridden"; }).pname == "hello-no-final-attrs-overridden";
+        expected = true;
+      })
     ];
 
   addEntangled = origOverrideAttrs: f: