Update Yarn 2 install command with a production flag - #123

Hi @NerOcrO,

First, all our apologies for getting back to you only now :-(

Your suggested change seems like a good idea.
Here are a few remarks though:

• In the official documentation, Yarn suggests to replace yarn install --production with yarn workspaces focus --all --production. The --all flag seems to be missing in your PR.
• We usually ask contributors to suggest their improvements upstream (in this case, it's here: https://github.com/heroku/heroku-buildpack-nodejs).
• The patch should now target lib/package_managers/yarn.sh, around line 429 (https://github.com/heroku/heroku-buildpack-nodejs/blob/03a871c36cf1359010d84626e11ae9511e95906c/lib/package_managers/yarn.sh#L429).

Thanks again <3