Update build workflow to build all workspaces

We split the build command for the old and new themes in #2503

See #2495

.github/workflows/build.yml

@@ -46,7 +46,7 @@ jobs:
       - name: yarn install and build
         run: |
           yarn
-          yarn build
+          yarn workspaces run build
 
       - name: Trim the repo down to just the source and built files
         run: |