Fixed application dir used for `COREPACK_HOME` in CI env

[colincasey]

The COREPACK_HOME directory was mistakenly set to use BUILD_DIR instead of HOME as the application root directory in the CI test environment causing failures when invoking pnpm or Yarn tests if those tools were installed via corepack.

---

Workaround

[!WARNING] The following is meant to be used as a temporary change to unblock anyone affected by this bug. Once this change is released, please remember to remove the workaround.

• Specify or replace the heroku/nodejs buildpack in app.json with the url of this branch:

{
  "environments": {
    "test": {
      "buildpacks": [
        {
          "url": "https://github.com/heroku/heroku-buildpack-nodejs#corepack-ci-fix"
        }
      ]
    }
  }
}