The `aio_monitoring_stable` CI job is triggered as a cronjob on the
master branch and its purpose is to run some e2e tests against the
deployed stable version of the docs web-app at https://angular.io/. In
order for the tests to be compatible with the deployed version of the
web-app (which gets deployed from the stable branch), the stable branch
is checked out in git as part of the CI job.

Previously, we only checked out the `aio/` directory from the stable
branch, leaving the rest of the code at master. This doesn't matter as
long as the commands used to run the tests do not rely on code outside
of `aio/`. However, it turns out that there _is_ code outside of `aio/`
that affects the executed commands: It is our vendored version of yarn
(in `third_party/github.com/yarnpkg/`), which overwrites the global yarn
installed on the docker image on CI and must match the version range
specified in `aio/package.json > engines`.

Using the yarn version checked out from the master branch with the
`aio/` code checked out from the stable branch can lead to failures
such as [this one][1].

This commit fixes the problem by checking out both the `aio/` and
`third_party/github.com/yarnpkg/` directories from the stable branch and
re-running the steps to overwrite the global yarn executable with our
own version from `third_party/github.com/yarnpkg/`. This ensures that
the version of yarn used will be compatible with the version range
specified in `aio/package.json > engines`.

NOTE:
We cannot checkout everything from the stable branch, since the CI
config (`.circleci/config.yml` from the master branch) may try to run
certain scripts (such as `.circleci/get-vendored-yarn-path.js`) that are
not available on the stable branch. Therefore, we should only check out
the necessary bits from the stable branch.

[1]: https://circleci.com/gh/angular/angular/567315

PR Close #34451