We keep a version of yarn in the repo, at
`third_party/github.com/yarnpkg/`. All CI jobs should use that version
for consistency (and easier updates).
Previously, the Windows jobs did not use the local version. They used
the version that came pre-installed on the docker image that we used.
This made it more difficult to update the yarn version (something that
we might want to do independently of updating other dependencies, such
as Node.js).
This commit fixes this by setting up the Windows CI jobs to also use the
local, vendored version of yarn.
PR Close#34384
George Kalpakas