feat: Use yarn v4

[timfish]

To have yarn v4 work with Volta I had to:

> npm install -g corepack
> corepack enable --install-directory ~/.volta/bin

[timfish]

For some reason the rollup build hangs at the end occasionally on macOS. I've seen this locally but can no longer reproduce it. It's strange because when it fails in a runner (and it doesn't fail in all of them), re-running always fails again.