Install latest builds from internal nightlies in beaker tests

[joshcooper]

Modules that rely on litmus use the puppet_agent module to install the latest
released and nightly versions of puppet-agent packages:

bundle exec rake 'litmus:install_agent[puppet7-nightly]'

Commits 025aa70 and b2e73a8 changed the default location where nightly
builds are installed from. Since we haven't documented how to do that, revert
those changes. Instead specify the internal sources in the test for the
situation where a platform has been recently added, but hasn't been released
yet, for example, macOS 15 ARM.

We will need to revisit how to install puppet*_nightly puppet core builds.

Fixes #758

Tested adhoc https://jenkins-platform.delivery.puppetlabs.net/view/puppet-agent/view/puppetlabs-puppet_agent%20module/view/ad-hoc/job/forge-module_puppetlabs-puppet-agent-module_intn-sys_pa-acceptance_7-nightly_to_8-nightly-adhoc/59/

Manual task redhat8

install task
Installed puppet-agent on tawny-mechanism.delivery.puppetlabs.net: success
Ensuring installed puppet-agent on tawny-mechanism.delivery.puppetlabs.net: success
Upgraded puppet-agent to latest puppet7 on tawny-mechanism.delivery.puppetlabs.net: success
Upgraded puppet-agent to puppet8 on tawny-mechanism.delivery.puppetlabs.net: success
.  works with version and install tasks
ssh connection to redhat8-64-1 has been terminated


Finished in 2 minutes 6.6 seconds (files took 1 minute 36.38 seconds to load)
1 example, 0 failures

Manual task macOS 15 ARM

install task
Installed puppet-agent on pix-arm64-macos15-5.nspooler-prod.puppet.net: success
.  works with version and install tasks
ssh connection to osx15-ARM64-1 has been terminated


Finished in 24.03 seconds (files took 23.02 seconds to load)
1 example, 0 failures

pix-arm64-macos15-5:~ root# /opt/puppetlabs/puppet/bin/puppet --version
8.11.0