[rubygems/rubygems] Fix `bin/rake spec:all` task

Only in CI, if two different test runs are started (like `bin/rake
spec:all` does), the second one would not install the dev version of
Bundler and would fail to start.

This commit makes it work the same locally and in CI.

https://github.com/rubygems/rubygems/commit/7a5ca6c40f

spec/bundler/spec_helper.rb

@@ -87,8 +87,6 @@ RSpec.configure do |config|
     # Don't wrap output in tests
     ENV["THOR_COLUMNS"] = "10000"
 
-    Spec::Helpers.install_dev_bundler unless ENV["CI"]
-
     extend(Spec::Builders)
 
     check_test_gems!

spec/bundler/support/builders.rb

@@ -235,6 +235,8 @@ module Spec
         FileUtils.rm_rf(base_system_gems)
         Spec::Rubygems.install_test_deps
       end
+
+      Helpers.install_dev_bundler unless pristine_system_gem_path.exist?
     end
 
     def update_repo(path, build_compact_index: true)