Force target Nailgun server to localhost

When using the external Nailgun client (the default on most
systems), make sure we always connect to localhost and not to
whatever the NAILGUN_SERVER environment variable might happen
to point to at the time the command (and the Nailgun client)
is executed.

Signed-off-by: Maxime Petazzoni <maxime.petazzoni@bulix.org>