Add NETRC to the list of default_pass_env.
This is used by pip when downloading package from an index needing authentication.

My usecase is a CI process needing to obtain authentication tokens and pass them to pip for it to be able to use the index. Installing the credentials in ~/.netrc would work but isn't always possible when using temporary tokens, or running CI jobs in parallel on the same host. The other supported way is to write a temporary netrc file, and point pip to it using NETRC environment variable. After the CI run is done, this netrc can be removed.

• [] ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation