Issue:
You would like to use a specific Yarn version in your builds, but this version might be different than the default on Bitrise.
Possible solutions:
On the MacOS stacks Yarn is installed via brew, so if you need the latest version, you can upgrade using brew command: brew upgrade yarn
If you need a specific Yarn version, you can switch between versions using brew switch yarn VERSION_NUMBER. If you are unsure, you can then verify the versions available to homebrew by running: brew list --versions yarn.