Migrate package manager to Yarn Modern (#628)

We were using Yarn Classic. While this is not deprecated, Yarn is moving
in a different direction with Yarn Modern (read more
[here](https://yarnpkg.com/migration/overview)) and we should align with
that. The migration mainly requires rebuilding our lockfile. We will
still use `node_modules` for now, though in the future we may want to
adopt the new [Plug'n'Play model](https://yarnpkg.com/features/pnp).

.testdriver/prerun.sh

@@ -3,7 +3,8 @@ cd ~/actions-runner/_work/testdriver/testdriver/
 brew install go
 brew tap scripthaus-dev/scripthaus
 brew install scripthaus
-npm install -g yarn
+corepack enable
+yarn install
 scripthaus run build-backend
 echo "Yarn"
 yarn