Fix yarn bug by standardizing yarn release executables to predictable names

Problem:

- Every few months yarn changes the name of the executable it asks you to check into your repository
- Previously it was `yarn-berry.js`, now it's `yarn-berry-$VERSION.cjs`
- Same with `plugin-workspace-tools`, which now ends with `.cjs`
- This is annoying because we want the executable to have the same name
   since we reference it scripts e.g. to check if yarn is installed, and because
   we don't want two developers checking in different yarn executables

Solution:

- Standardize on the previous names (`.js`, no version in filename), so we
   always know that yarn is at `.yarn/releases/yarn-berry.js` and
   `plugin-workspace-tools.js` is always at `.yarn/plugins/@yarnpkg`
- The `setup.sh` script is convoluted -- that's partially because it needs
   to be run in a number of different contexts (this repo on the host, this
   repo in a container, and our cloud repo in both as well), and also because
   it runs as the docker entrypoint so needs to be efficient

Author: milesrichardson