fix(plugin-pnp): wait for unplugging to finish before finalizing

[merceyz]

What's the problem this PR addresses?

There is a race condition in the PnP linker where it can end up trying to read the unplugged folder before it is created.

Fixes https://github.com/yarnpkg/berry/runs/5705475812?check_suite_focus=true#step:5:154

Error: ENOENT: no such file or directory, scandir '/tmp/xfs-f4bc4a3d/.yarn/unplugged'

How did you fix it?

Wait for the unplugging to complete before calling finalizeInstallWithPnp

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.