[rubygems/rubygems] Fix Bundler daily CI

A default Bundler copy with vendored net-http is causing a Bundler spec
to end up loading that copy instead of the copy of Bundler under test.
This is because of vcr, but we don't really need vcr in the command
that's failing, so I avoided loading it.

rubygems/rubygems@304ab6e1b9

spec/bundler/runtime/self_management_spec.rb

@@ -28,7 +28,7 @@
       expect(out).to include("Bundler #{Bundler::VERSION} is running, but your lockfile was generated with #{previous_minor}. Installing Bundler #{previous_minor} and restarting using that version.")
 
       # It uninstalls the older system bundler
-      bundle "clean --force"
+      bundle "clean --force", artifice: nil
       expect(out).to eq("Removing bundler (#{Bundler::VERSION})")
 
       # App now uses locked version