Prefer local packages when present in cache
Prefer local packages when present in cache

This change updates the `yarn install` step to use the
`--prefer-offline` argument. This avoids reinstalling already-present
packages when the local cache has them.

heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs