Prune package.json workspaces fixed; process trees await task completion

<!-- Release notes generated using configuration in .github/release.yml at v2.9.15-canary.1 -->

What's Changed

Changelog

• chore: Release 2.9.13 by @anthonyshew in https://github.com/vercel/turborepo/pull/12803
• release(turborepo): 2.9.14 by @github-actions[bot] in https://github.com/vercel/turborepo/pull/12805
• fix: Prune package.json workspaces by @anthonyshew in https://github.com/vercel/turborepo/pull/12808
• fix: Wait for process trees before task completion by @anthonyshew in https://github.com/vercel/turborepo/pull/12809

Full Changelog: https://github.com/vercel/turborepo/compare/v2.9.13-canary.1...v2.9.15-canary.1