Use conventional `NETRC` environment variable name for netrc credential file

The previously used environment variable name required the downstream
tool (release/make-release/automatic-branch-cut) to set HOME and
symlink the netrc file. If we instead use the standard-ish NETRC
environment variable name, the file should be picked up by Python's
requests library and branch.py will configure git to use the resolved
credentials.

Reverts the revert (8d2d8fec) of the
original.

Bug: T373441
Depends-On: release!110 (closed)