(PA-7432) Install server packages from internal nightlies

[joshcooper]

Previously, we installed puppet-agent and puppetserver packages from public nightlies, but those are no longer updated. Switch to using internal nightlies instead, with these caveats:

• I didn't change the nightly_apt_repo_url, because we only test with RHEL puppetservers during upgrade testing.

• This change doesn't affect agent-only nodes. Those will be handled in PA-7431

I manually tested this using:

$ cd acceptance
$ bundle install
$ bundle exec beaker-hostgenerator debian12-64default.a-redhat9-64mdca --hypervisor abs > hosts.yaml
$ bundle exec beaker init -h hosts.yaml -o options.rb --debug
$ bundle exec beaker provision --debug
$ bundle exec beaker exec ./pre_suite --debug
$ bundle exec beaker exec ./tests/test_upgrade_puppet7_to_puppet8.rb --debug