When the test are run npm read the ~/.npmrc and ./.npmrc files and set some environment variables starting with `npm_`.
When the test create temporary folders and created a `.npmrc` the values there are ignored as the environment variables take precedence.
This commit remove this environment variable (from the local `process.env`) before starting the integrations test and restore them after. This way the `./.npmrc` files created in temp directory for the test are correctly used.
ce3841f553