chore: use corepack to install yarn

[crazy-max]

related to docker/bake-action#98

Corepack creates shims next to the node binary, e.g. yarn in our case. The first time we invoke yarn, it knows which version we want by looking at the package.json file:

actions-toolkit/package.json

Line 33 in ae20b6f

"packageManager": "yarn@3.6.3",

and is able to download that version and cache it. This avoids having the package manager in the working tree.