test(ci): use new build target after removing old-yarn prepare

since yarn doesn't use `prepare` as a lifecycle script anymore, but
seeing it means you expect it to run on install, it is best to avoid the
name so it is clear you have to call it yourself as needed

.github/workflows/ci.yml

@@ -65,4 +65,4 @@ jobs:
         uses: ./.github/actions/setup
 
       - name: Build package
-        run: yarn prepare
+        run: yarn build