(maint) Start shipping to puppet6 nightlies
(maint) Start shipping to puppet6 nightlies

In the lead up to the puppet6 release, we have created a second nightly
repo for v6 components. This commit enables puppetserver's master branch
to start shipping builds to that repo instead of the puppet5 repo.

puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppet-server,puppetlabs/puppetserver