(PA-8445) Use Artifactory as sole gem source

[mhashizume]

Prior to this, our internal Artifactory mirror, which hosts Puppet Core releases, was added as a gem source, but came after the default location of rubygems.org.

This commit updates the nightly gem test workflow to add Artifactory and remove rubygems.org in order to make sure that the Puppet nightly gem correctly installs the latest Facter as its dependency from Artifactory instead of the outdated last publicly-released Facter from rubygems.org.

[mhashizume]

Successfully tested here: https://github.com/puppetlabs/puppetlabs-augeas_core/actions/runs/26774529279/job/78922536306

Run curl https://artifactory.delivery.puppetlabs.net/artifactory/internal_nightly__local/downloads/gems/puppet8-nightly/puppet-latest.gem --output puppet.gem --location
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
 41 1382k   41  569k    0     0   613k      0  0:00:02 --:--:--  0:00:02  613k
100 1382k  100 1382k    0     0  1400k      0 --:--:-- --:--:-- --:--:-- 1400k
https://artifactory.delivery.puppetlabs.net/artifactory/api/gems/rubygems/ added to sources
https://rubygems.org/ removed from sources
Successfully installed semantic_puppet-1.1.1
Successfully installed scanf-1.0.0
Successfully installed hocon-1.4.0
Successfully installed puppet-resource_api-1.9.2
Successfully installed multi_json-1.18.0
Successfully installed locale-2.1.5
Successfully installed getoptlong-0.2.1
Successfully installed fast_gettext-3.1.0
Successfully installed thor-1.5.0
Successfully installed facter-4.19.0
Successfully installed deep_merge-1.2.2
Successfully installed concurrent-ruby-1.3.6
Successfully installed puppet-8.19.0.10.g26d883a
13 gems installed

Note that Facter 4.19.0 is installed now instead of 4.10.0