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/tools/build-managers/scons/default.nix

@@ -1,12 +1,12 @@
-{ callPackage }:
+{ callPackage, python2Packages }:
 
 let
   mkScons = args: callPackage (import ./common.nix args) { };
 in {
-  scons_3_0_1 = mkScons {
+  scons_3_0_1 = (mkScons {
     version = "3.0.1";
     sha256 = "0wzid419mlwqw9llrg8gsx4nkzhqy16m4m40r0xnh6cwscw5wir4";
-  };
+  }).override { python3Packages = python2Packages; };
   scons_latest = mkScons {
     version = "3.1.2";
     sha256 = "1yzq2gg9zwz9rvfn42v5jzl3g4qf1khhny6zfbi2hib55zvg60bq";