Emergency build fix until I can unravel how sky_snapshot runs

@abarth @collinjackson

sky/packages/workbench/pubspec.yaml

@@ -6,6 +6,8 @@ homepage: https://github.com/domokit/sky_engine/tree/master/sky/packages/workben
 dependencies:
   sky: any
   sky_tools: any
+  # This is a hack until sky_snapshot no longer uses workbench's packages directory.
+  playfair: any
 dependency_overrides:
   material_design_icons:
     path: ../material_design_icons