nix_modules: generate clan.py in top-level shell

Generally I don't think dev shells are the right place to trigger generation of files.
This should be part of the formatter and pre-commit hook

@hsjobeki

devShell.nix

@@ -44,6 +44,9 @@
           # vendoring / needed for impure tests
           ln -sfT ${self'.packages.clan-cli.nixpkgs} "$PRJ_ROOT/pkgs/clan-cli/clan_lib/nixpkgs"
           ln -sfT ${inputs.nix-select} "$PRJ_ROOT/pkgs/clan-cli/clan_lib/select"
+
+          # Generate classes.py from schemas
+          ${self'.packages.classgen}/bin/classgen ${self'.legacyPackages.schemas.clan-schema-abstract}/schema.json $PRJ_ROOT/pkgs/clan-cli/clan_lib/nix_models/clan.py
         '';
       };
     };