Install latest builds from internal nightlies in beaker tests

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.

Author: joshcooper

task_spec/spec/acceptance/init_spec.rb

@@ -48,6 +48,16 @@ def latest_platform_list
     }x
   end
 
+  # This method lists sources to be used when installing packages that haven't been released yet (see above).
+  def latest_sources
+    {
+      'yum_source'     => 'https://artifactory.delivery.puppetlabs.net:443/artifactory/internal_nightly__local/yum',
+      'apt_source'     => 'https://artifactory.delivery.puppetlabs.net:443/artifactory/internal_nightly__local/apt',
+      'mac_source'     => 'https://artifactory.delivery.puppetlabs.net:443/artifactory/internal_nightly__local/downloads',
+      'windows_source' => 'https://artifactory.delivery.puppetlabs.net:443/artifactory/internal_nightly__local/downloads',
+    }
+  end
+
   it 'works with version and install tasks' do
     case target_platform
     when latest_platform_list
@@ -57,7 +67,7 @@ def latest_platform_list
       # Install an puppet8 nightly version
       results = run_task('puppet_agent::install', 'target', { 'collection' => 'puppet8-nightly',
                                                               'version' => 'latest',
-                                                              'stop_service' => true })
+                                                              'stop_service' => true }.merge(latest_sources))
 
       results.each do |result|
         logger.info("Installed puppet-agent on #{result['target']}: #{result['status']}")