scons: Add passthru.py2 for backward compatibility

Not all packages build with Python 3, see #75877. The goal is to get rid
of Python 2 but this approach ensures a smoother transition.

pkgs/development/libraries/serf/default.nix

@@ -9,7 +9,7 @@ stdenv.mkDerivation rec {
     sha256 = "1k47gbgpp52049andr28y28nbwh9m36bbb0g8p0aka3pqlhjv72l";
   };
 
-  nativeBuildInputs = [ pkgconfig scons ];
+  nativeBuildInputs = [ pkgconfig scons.py2 ];
   buildInputs = [ apr openssl aprutil zlib libiconv ]
     ++ stdenv.lib.optional (!stdenv.isCygwin) kerberos;