[ruby/rubygems] Clear BUNDLE_COOLDOWN for the RubyGems test suite

Cooldown resolution reads Bundler's settings now, so a developer with one
configured in their shell saw it applied to the fixtures: four install
command tests and two update command tests failed under
BUNDLE_COOLDOWN=60.

ruby/rubygems@42e80d83ba

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: 2 people

test/rubygems/helper.rb

@@ -397,6 +397,9 @@ def setup
     ENV["BUNDLE_CONFIG"] = nil
     ENV["BUNDLE_USER_CONFIG"] = nil
     ENV["BUNDLE_USER_HOME"] = nil
+    # Cooldown resolution reads Bundler's settings, so a developer running the
+    # suite with one configured would otherwise see it applied to the fixtures.
+    ENV["BUNDLE_COOLDOWN"] = nil
     ENV["RUBYGEMS_PREVENT_UPDATE_SUGGESTION"] = "true"
 
     @current_dir = Dir.pwd