enforce-registry does not work well with Yarn

[hdpe]

Yarn's registry selection behaviour is surprising and detection of the registry is not working when the registry is set in .npmrc.

.npmrc takes priority over .yarnrc when Yarn comes to pick a registry during install. However, enforce-registry does not detect this .npmrc registry.

The relevant Yarn issues are probably yarnpkg/yarn#6700 and yarnpkg/yarn#3683.

Scenario:

• .npmrc contains registry=http://x.com
• .yarnrc contains no registry

Actual Behaviour:

• yarn install calls npm scripts with npm_config_registry=https://registry.yarnpkg.com/ in the environment
  • however, yarn install would actually use http://x.com to download artifacts
• enforce-registry thinks the registry used is https://registry.yarnpkg.com/ (because this npm_config_registry environment variable overrides the .npmrc value when parsed by rc).

Expected Behaviour

• enforce-registry thinks the registry used is http://x.com