packaging: Expose the local overrides via passthru.

default.nix

@@ -119,7 +119,9 b' let'
       # TODO: johbo: Make a nicer way to expose the parts. Maybe
       # pkgs/default.nix?
       passthru = {
-        inherit myPythonPackagesUnfix;
+        inherit
+          pythonLocalOverrides
+          myPythonPackagesUnfix;
         pythonPackages = self;
       };