Add Windows netrc support?

[benjamin-bader]

We have an org-private Artifactory server that we're using with rules_jvm_external, and would like to standardize on .netrc files for providing credentials. The problem, I've discovered, is that the get_home_netrc_contents explicitly does not handle Windows. It kind of makes sense given that the file is non-standard and comes from GNU, but some common tools (git for example) have some kind of support for _netrc on Windows.

The usual algorithm for locating the file on Windows seems to be:

1. Look for a HOME environment variable
2. Failing that, find the USERPROFILE environment variable
3. Whichever is found, look for a file named _netrc

Would you be open to a PR that adds this support?