We keep a version of yarn in the repo, at
`third_party/github.com/yarnpkg/`. All CI jobs (including Windows ones)
should use that version for consistency (and easier updates). The path
to the actual `yarn.js` script, however, changes depending on the
version (e.g. `third_party/github.com/yarnpkg/v1.21.1/...`).
(NOTE: The Windows jobs are currently not using this local version, but
that should be fixed in a subsequent commit.)

Previously, when updating the local version of yarn, we would
potentially have to update the path in several places.

This commit addresses the problem by adding a Node.js script that infers
the correct path. The script can be used in all places where we need to
use the local version of yarn (including both Linux and Windows CI
jobs), thus eliminating the need to update the path in several places.

PR Close #34384