Build: Disable yarn immutable install by default during E2E tests

[gaetanmaisse]

What I did

When running the E2E tests we are doing a fresh install of Yarn using the latest version; as of Yarn 3.0.0 rc1 a behaviour on dep install has been updated:

The enableImmutableInstalls will now default to true on CI (we still recommend to explicitly use --immutable on the CLI).
You can re-allow mutations by adding YARN_ENABLE_IMMUTABLE_INSTALLS=false in your environment variables.

So I updated the Yarn config used in the E2E to be able to properly bootstrap the different projects.

For details, see https://github.com/yarnpkg/berry/blob/master/CHANGELOG.md#300-rc1

How to test

• CI should be back to 🟢

[gaetanmaisse]

@shilman, self-merging to fix CircleCI checks on next, other CI improvements and fixes will be done with #14522