Bump @wp-playground/cli from 3.1.15 to 3.1.21 in /env (#335)

Bumps [@wp-playground/cli](https://github.com/WordPress/wordpress-playground) from 3.1.15 to 3.1.21.
- [Release notes](https://github.com/WordPress/wordpress-playground/releases)
- [Changelog](https://github.com/WordPress/wordpress-playground/blob/trunk/CHANGELOG.md)
- [Commits](WordPress/wordpress-playground@v3.1.15...v3.1.21)

---
updated-dependencies:
- dependency-name: "@wp-playground/cli"
  dependency-version: 3.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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