fyi, for anyone who comes across this, i solved it by breaking sitemap into it's own task:

sitemap": {
  "dependsOn": ["build"],
  "outputs": ["public/robots.txt", "public/sitemap*.xml"]
},

and then a deploy task which both builds and sitemaps:

"deploy": {
  "dependsOn": ["build", "sitemap"],
  "outputs": []
},

Also note that we need to change name of postbuild script to sitemap.

For extra clarification, you will also have to change your build command in your app deployment settings. I'm using Vercel, so now my build command looks like this: cd ../.. && npx turbo run deploy --filter=[my-app-name]...