Restore .npmrc to its original state

[pvdlg]

The plugin create a local .npmrc file for authentication.
After each step (verifyConditions, prepare, publish, addChannel) we should restore the local .npmrc to its original state:

• If there was no .npmrc file we should delete the one we have created
• If there was a .npmrc file we should restore it

See semantic-release/semantic-release#1109

[pvdlg]

It seems there is some interest from npm to support authentication via environment variables. That would avoid to have to modify the .npmrc.
The feature has been implemented but it doesn't works: npm/cli#8 and https://npm.community/t/cannot-set-npm-config-keys-containing-underscores-registry-auth-tokens-for-example-via-npm-config-environment-variables/233

[semantic-release-bot]

🎉 This issue has been resolved in version 5.3.0 🎉

The release is available on:

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

Your semantic-release bot 📦🚀