Once nexus3 push is complete, unset alt_nexus_url

create-netrc uses the value of alt_nexus_url if it's set. We need to
remove this value so that the correct credentials are used for
pushing logs at the end of the job.

Change-Id: Ie3704f3e83226088e6b2d700c0e3ee2c5318e257
Signed-off-by: Eric Ball <eball@linuxfoundation.org>