make-derivation.nix: Return mkDerivation as an attribute

pkgs/stdenv/adapters.nix

···
 
 let
   # N.B. Keep in sync with default arg for stdenv/generic.
-  defaultMkDerivationFromStdenv = import ./generic/make-derivation.nix { inherit lib config; };
+  defaultMkDerivationFromStdenv = stdenv: (import ./generic/make-derivation.nix { inherit lib config; } stdenv).mkDerivation;
 
   # Low level function to help with overriding `mkDerivationFromStdenv`. One
   # gives it the old stdenv arguments and a "continuation" function, and

pkgs/stdenv/generic/default.nix

···
 
 , # The implementation of `mkDerivation`, parameterized with the final stdenv so we can tie the knot.
   # This is convient to have as a parameter so the stdenv "adapters" work better
-  mkDerivationFromStdenv ? import ./make-derivation.nix { inherit lib config; }
+  mkDerivationFromStdenv ? stdenv: (import ./make-derivation.nix { inherit lib config; } stdenv).mkDerivation
 }:
 
 let

pkgs/stdenv/generic/make-derivation.nix

···
   (derivation (derivationArg // optionalAttrs envIsExportable checkedEnv));
 
 in
-  mkDerivation
+{
+  inherit mkDerivation;
+}