Local cache of node_modules doesn't work with yarn workspaces

See https://stackoverflow.com/questions/55890275/aws-codebuild-does-not-work-with-yarn-workspaces for a description of the problem.

Start with the linked Stack Overflow description and inspect the repository's CodeBuild samples for how Yarn workspaces and the local node_modules cache are configured. Determine the expected cache behavior and identify a reproducible build before assessing what would count as a fix.

Written by the indexing model from the issue text.