Handle NPM_CONFIG_GLOBALCONFIG when present

[chaitan94]

Summary

This pull request is to address issue #5543
I hope it helps! Any feedback to improve the code is welcome.

Also, I am not sure if this helps resolve #4451 as well.

Test plan

• Added an appropriate unit test and tested using node_modules/jest/bin/jest.js -i __tests__/registries/npm-registry.js
• Manually tested with the following steps:
  1. Created a file /tmp/customrc with the content ANSWER=42
  2. Built the yarn binary yarn build
  3. Ran NPM_CONFIG_GLOBALCONFIG=/tmp/customrc ./bin/yarn config get ANSWER
  4. Verified that the output is 42 as expected.

[phgn0]

Can we please get this PR reviewed? It's solving a very real config bug.