Preview builds were setup to install correctly on CircleCI prior to
the Yarn v3 upgrade, but that integration broke with that upgrade. The
Yarn and CircleCI configuration has been updated to fix this.
The `.yarnrc.yaml` file has been updated to configure the GitHub
registry but leave it disabled by default. It can be enabled
dynamically using an environment variable. This lets us switch between
registries without updating the file.
The new workflow is documented here: https://github.com/MetaMask/core/pull/1481
Unlike Yarn Classic, Yarn Berry will not verify that the current version
of Node being used matches the `engines` field in `package.json`.
However, someone has written a plugin, [`engines`][1], that provides
this missing functionality, and this commits adds that plugin. This
should ensure that all developers are using the correct version of Node.
(In addition to checking the current Node version, the plugin also
checks the current Yarn version.)
[1]: https://github.com/devoto13/yarn-plugin-engines
