flake: set Nix Qt environment variables in development shell

allows applets to run standalone via ``python -m ...`` without requiring the Nix Qt wrapper

flake.nix

@@ -447,6 +447,8 @@
         ];
         shellHook = ''
           export LIBARTIQ_SUPPORT=`libartiq-support`
+	  export QT_PLUGIN_PATH=${pkgs.qt5.qtbase}/${pkgs.qt5.qtbase.dev.qtPluginPrefix}
+          export QML2_IMPORT_PATH=${pkgs.qt5.qtbase}/${pkgs.qt5.qtbase.dev.qtQmlPrefix}
         '';
       };