Fallbacks to npm when COREPACK_NPM_REGISTRY is set

[arcanis]

It's been mentioned a couple of times that Yarn publishing its versions on its own was inconvenient for people using private npm mirrors. I'd prefer to keep using our store by default, but it makes sense to offer a way to fallback to the npm store when requested. This diff changes the logic so that we use the @yarnpkg/cli-dist package when the COREPACK_NPM_REGISTRY variable is set. It should be backward-compatible, since the registry field is still the same.

Fixes #337

[aduh95]

Note that means that corepack use yarn@latest will now work, but only when COREPACK_NPM_REGISTRY is set 😅

[arcanis]

I'm starting to desentangle that in #340, although I'll also need to tweak @yarnpkg/cli-dist so that its tags match the ones from /repo (latest currently points to canary on our npm releases, whereas we'd want them to point to stable instead).

[PayBas]

I feel like I'm missing something here:

USER root

...

ARG NODEJS_VERSION='18.19.0'
ENV PATH "/opt/node/bin:$PATH"
RUN  mkdir -p /opt/node \
  && tar -xzf /installers/node-v${NODEJS_VERSION}-linux-x64.tar.gz --directory=/opt/node --strip-components=1 --no-same-owner

USER jenkins

# Configure npm & install+configure Yarn.
ARG YARN_VERSION='4.1.0'
ARG NPM_REGISTRY_URL='https://nexus.corp.internal/repository/npmjs-proxy/'
ENV COREPACK_NPM_REGISTRY $NPM_REGISTRY_URL
ENV NPM_CONFIG_PREFIX ${HOME}/.npm-global
ENV PATH ${NPM_CONFIG_PREFIX}/bin:$PATH
RUN  npm config set registry $NPM_REGISTRY_URL \
  # Node 18.19.0 ships with corepack 0.22.0, but we need corepack 0.24.0+
  && npm install --global corepack@latest \
  && corepack enable \
  && corepack --version \
  && corepack install --global yarn@${YARN_VERSION} \
  && yarn config set --home npmRegistryServer $NPM_REGISTRY_URL

Results in:

0.24.1
Installing yarn@4.1.0...
Internal Error: Error when performing the request to https://repo.yarnpkg.com/4.1.0/packages/yarnpkg-cli/bin/yarn.js; for troubleshooting help, see https://github.com/nodejs/corepack#troubleshooting
    at ClientRequest.<anonymous> (/home/jenkins/.npm-global/lib/node_modules/corepack/dist/lib/corepack.cjs:42208:16)
    at ClientRequest.emit (node:events:517:28)
    at TLSSocket.socketErrorListener (node:_http_client:501:9)
    at TLSSocket.emit (node:events:517:28)
    at emitErrorNT (node:internal/streams/destroy:151:8)
    at emitErrorCloseNT (node:internal/streams/destroy:116:3)
    at process.processTicksAndRejections (node:internal/process/task_queues:82:21)

@arcanis I'm not sure why corepack is still trying to connect to repo.yarnpkg.com instead of respecting the COREPACK_NPM_REGISTRY.
This is a corporate machine with no direct internet access ;).

[plumpNation]

@PayBas did you ever find out why this was happening?