heroku-buildpack-nodejs: Yarn cache does not work by default

Hi, the docs

https://devcenter.heroku.com/changelog-items/1082 https://devcenter.heroku.com/articles/nodejs-support#cache-behavior

suggests that yarn caching works by default. My builds consistently showed however that only node_modules was being cached, which is insufficient for yarn workspaces and also inconsistent with what the docs seems to specify above.

I tried setting cacheDirectory to the output of heroku run yarn cache dir but alas no avail.

After much digging I found out the buildpack sets the yarn cache dir to a temporary location: https://github.com/heroku/heroku-buildpack-nodejs/pull/459

I tried working around this by having the following in package.json:

    "heroku-prebuild": "export YARN_CACHE_FOLDER=/app/.cache/yarn/v4"

But this did not help the problem.

What would really help would have been the following PR which was never merged: https://github.com/heroku/heroku-buildpack-nodejs/pull/615