Allow setting alternate URLs to download yarn versions

[onigoetz]

Hello,

I'm using Renovate to update Yarn in hundreds of repositories.
Renovate leverages corepack to download Yarn which seems to result in our source IP being banned because we download Yarn too often.

I operate a Nexus Repository from which I could download a release only once and then use it from there as a cache, but apparently there is no way to provide an alternative baseURL for Yarn?

I only see COREPACK_NPM_REGISTRY but that doesn't apply for Yarn.

! Corepack is about to download https://repo.yarnpkg.com/4.17.1/packages/yarnpkg-cli/bin/yarn.js

Is it possible to introduce COREPACK_YARN_MIRROR ?

COREPACK_YARN_MIRROR=https://repo.yarnpkg.com/

I would be happy to send a Pull Request should this be accepted.