Deleted files are still present in CI runs

[spalladino]

In this run we hit a CI error since a file that was deleted in the PR was present on the CI run. We believe it's because yarn-project-base copies the entire project into the docker image, and then each package builds off of that by COPYing over again the latest version. But if yarn-project-base didn't get rebuilt due to no changes, then it will carry the old set of files (which includes the deleted one), and overwriting the entire project in each package built only overwrites existing files.

We should see if we can remove the COPY . . step from yarn-project-base's Dockerfile, or if we can tweak subsequent COPY . . steps so that it properly deletes files.

As a workaround, triggering a rebuild of yarn-project-base fixes things.