fix: support npmrc for custom-version env variable

[felixrieseberg]

Reopened https://github.com/electron/get/pull/219 here because CircleCi didn't want to run builds from forks

Fixes following problems:

• It is not possible to define CUSTOM_VERSION env variable via .npmrc together with others.
• it was not picking up customDir and nightlyMirror from the mirrorConfig properly

[ddramone]

@felixrieseberg looks sane to me.

Is it easy to add a test for this?

Yes, I can do that

[ddramone]

Hi @felixrieseberg

I added a test in this commit

I'm not sure how can I push it to this PR since it was created by you.

[electron-bot]

:tada: This PR is included in version 2.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: