Configure http proxy before calling Yarn

[rarkins]

Describe the proposed change(s).

Yarn apparently does not recognize HTTP_PROXY environment variables, and instead requires explicit configuration.

It can be configured like this:

      yarn config set proxy http://proxy.domain.tld:port
      yarn config set https-proxy http://proxy.domain.tld:port

I did not find any "noproxy" support.

We should perhaps set them globally though, so as not to modify any .yarnrc.yml in the repo if it exists.

If the HTTP_PROXY/HTTPS_PROXY strings include credentials in the URL we should make sure we redact them in logs.

Docs: https://yarnpkg.com/configuration/yarnrc#httpProxy

Testing this will require a local http proxy, and Renovate will forward all its requests through it too due to the HTTP_PROXY/HTTPS_PROXY variables being set.

[rarkins]

Closed by #27794