message: fix(express): use yarn to run build script
  `tsc` is now installed in the root node_modules, so npm can't find it
  anymore. Instead, use yarn to run `tsc` since it knows about the
  binaries installed in the root.
diff: "unit-test": "nyc -- mocha --opts test/mocha.opts \"test/unit/**/*.ts\"",
  "integration-test": "nyc -- mocha --opts test/mocha.opts \"test/integration/**/*.ts\"",
  "clean": "rm -rf dist/*",
  - "prepare": "npm run build",
  + "prepare": "yarn build",
  "audit": "if [ \"$(npm --version | cut -d. -f1)\" -ge \"6\" ]; then npm audit; else echo \"npm >= 6 required to perform audit. skipping...\"; fi",
  "lint": "eslint 'src/**/*.ts' 'test/**/*.ts' || true",
  "build": "tsc",
  