Try to use Gem::Installer for default gems in Bundler's example

bundler/spec/support/helpers.rb

@@ -320,10 +320,20 @@ def system_gems(*gems)
     def install_gem(path, install_dir, default = false)
       raise "OMG `#{path}` does not exist!" unless File.exist?(path)
 
-      args = "--no-document --ignore-dependencies --verbose --local --install-dir #{install_dir}"
-      args += " --default" if default
-
-      gem_command "install #{args} '#{path}'"
+      require "rubygems/installer"
+      installer = Gem::Installer.at(
+        path,
+        document: [],
+        ignore_dependencies: true,
+        verbose: true,
+        domain: :local,
+        install_dir: install_dir,
+      )
+      installer.install
+      if default
+        File.delete installer.spec_file
+        installer.write_default_spec
+      end
     end
 
     def with_built_bundler(version = nil, &block)